Priority 1: Not wasting resources
This means you are always able to generate new resources, and never "capped" on any resources. Frost dks have 5 "resources" to worry about.
Runes regen over time, without you needing to do anything. The rate changes based off your haste, but what's important is that the regen stops when you are rune capped. The solution to this is to avoid, whenever possible, getting rune capped. Think of dks as having 3 runes, not 6. We have 1 frost, 1 unholy, and 1 death. When a rune is charged 50%, you can spend that 50% on an ability, but the rune continues to recharge. However, when a rune is 100% charged, you have both frost runes up, it stops recharging. To avoid this is simple, always spend runes before they reach 100%.
To spend unholy runes, you generally use Plague strike if the target is missing blood plague, or obliterate if it isn't.
To spend frost runes, you use soul reaper if it's off CD, and the target is sub 35%, and you otherwise use obliterate.
To spend death runes is the same as spending frost runes.
Runic power is generated while using abilities, namely abilities that use up runes, as well as horn of winter. Runic power is capped at 100, and the ability that generates the most is obliterate, giving you 24 runic power. To avoid capping runic, always use it before it reaches 100%. That means don't use obliterate at higher than 76 runic, and don't use any other rune abilities at higher than 88 runic.
To spend runic, use frost strike.
Rime procs are generated using obliterate, and give you a free howling blast. You can hold onto a rime proc as long as you want, assuming you don't wait the entire duration, or use obliterate during it, which has a chance of overwriting the proc. In practice, you rarely, if ever, go the full duration without having a free GCD to use it, so just don't obliterate with rime up.
To spend rime, use howling blast.
Tier 5 talent procs:
Your tier 5 talent will proc on using frost strike, and depending on which you choose, you can change your play-style.
RE: Don't use frost strike when you don't have a rune below 50%
RC: Don't use frost strike when you have any rune at 100%
BT: Don't use frost strike when you have 11 or more charges.
Killing Machine procs:
Killing Machine procs on melee hits, and is used up by turning your next obliterate or frost strike into a crit. Killing machine is also the interesting proc, because you want to use it up as soon as possible to avoid overriding a proc, but you can't stop meleeing, and you also want to wait to use it on an obliterate instead of a frost strike. More on this later.
Priority 2: Getting as many obliterates to crit as possible.
With average values of mastery, obliterate will hit about twice as hard as frost strike. It will also crit twice as hard. For this reason, "wasting" killing machine procs on frost strikes is more harmful to your damage then ever before.
Interestingly, when you are not wasting any resources by using the above methods, and the boss isn't about to die in 5 seconds or less, you actually lose no damage by holding runes, or runic, or procs. If it allows you to increase your crit rate on obliterate, it is a damage gain.
Getting obliterate to crit as often as possible involves sitting on runes. Simply don't spend them unless you are about to lose a resource, via over-capping, or tier 5 talents, or you have killing machine procs.
The oposite treatment for frost strike is to never use frost strike when killing machine is up, even if you have the runic for it. Only use frost strike when you are about to overcap runic, or you don't have killing machine.
Sadly, more often then not killing machine will proc while we're spamming frost strike, and get used up a quarter of a second later. However, most of the time we have free GCDs, and the best way to use these is to spread them out in-between frost strikes. Instead of spamming frost strike when we have 80 runic power, hit it once, and 1.1 seconds later, hit it again. Another 1.1 seconds later, tap it again. You'll almost never use up KM procs prematurely this way, and will have ample time to change to obliterate, should killing machine proc.
Mastering rime procs
Rime procs are free damage, and should never be ignored. However, they can be put off. Rime, along with to a lesser degree horn of winter, are the main abilities we use on a regular basis, which do not use up KM procs. This means that we can spam them, HoW and then rime for example, without worrying about wasting a KM proc. However, if we use frost strike after rime or HoW, any procs generated during that GCD will most likely get used up on the frost strike. In addition, while obliterate procs more rime, frost strike doesn't. For this reason, it's normally optimal to wait on a rime proc until you've used up as many of the staggered frost strikes as possible, and then to obliterate soon after. If killing machine procs during the frost strike taping, with rime up, you want to hit the rime proc, and then obliterate.
Mastering diseases with plague leech
Diseases are invaluable for death knights, especially frost, but not for their damage. Diseases do make obliterate much stronger though, and you should never use obliterate without both diseases up. However, missing blood plague when you frost strike is ok, most of the time. We have two ways of applying diseases, and two ways of losing them. We can apply them with outbreak, on a 1 minute cooldown, or with plague strike and howling blast, each costing a rune. We lose them if they are not reapplied for 30 seconds, or if we use plague leech to turn them into a free rune. However, plague leech is not always optimal.
Once every minute, when outbreak is no longer on cooldown, or is a few seconds from coming up, you can follow a plague leech with outbreak for a free disease refresh and rune. However, if outbreak is on cooldown, the best way to reapply diseases is to try to get a rime proc in the 10 seconds before the diseases falls off. If you do, you want to hold off on using the rime proc until blood plague nearly runs out, use plague leech, reapply frost fever with rime, and then use the free rune on a plague strike, which you would of have to of done anyway.
Mastering your GCDs
Being able to wait, and still increase damage, requires having the time available. That time is labeled free GCDs by the wow community, and what it means is that during your normal rotation, you're not able to use an ability every second. Let's say you have 30 second "rotation" as a dk, starting when you apply your diseases. During that 30 seconds, you're going to regen each rune 5 times, for 15 runes total. Those 15 runes are going to give you 7 obliterates, or 7 GCDs worth of attacks. Those 7 obliterates net you 168 runic, or 8 frost strikes, and another 8 GCDs. That's 15 GCDs during a 30 seconds period. Sometimes procs will fill up the other 15, sometimes they will not. Waiting requires smart play, the ability to know if you have enough GCDs left to use up all your abilities before you get capped anywhere, and whether or not you can afford to wait, or for how long. You can almost always tap FS instead of spamming it for a gain, unless you are truly GCD capped, which often happens when you get a significant haste buff (sinestra p3) or lust/heroism is up. Knowing if you can afford to wait 3 seconds on an obliterate is a judgment call. If you find yourself over-capping on anything, you're waiting too much, if you find yourself never over-capping, you're waiting a good amount. The longer you can wait, without over-capping, the more skilled you are at this priority.
On occasion, you will receive an extra ability, not typically present in a death knight's arsenal. This ability is wild mushroom: plague, and at it's core is a longer version of unholy blight with a longer cooldown. Running a 3 minute cooldown, and a 30 second duration, this placeable mushroom sends diseases to everything within 10 yards, and ticks every 3 seconds. At first glance, you can simply use it to avoid the need to plague strike, starting a fight with outbreak, using WM:P when diseases fall off, and then outbreak is up again for the next round. The ticks of the diseases also make it interesting to combine with plague leech. Leech off the first set of diseases for a free rune, and then pop WM:P. 25 seconds later, leech it again, only to have WM:P reapply without you even needing to hit a button. Because of the 3 second ticks, you may need to hold off on an obliterate for up to 3 seconds, but hey, free rune! When that set of diseases runs low, simply go back to your normal diseases spreading abilities until WM:P comes off cooldown.
How long is it worth waiting?
There are two cases where waiting can be a damage gain.
With enough runes to obliterate, but no KM:
Assuming you have not wasted any resources, there is no penalty for waiting. You lose absolutely nothing, no runic overcapping, no T5 procs wasted, no rimes overridden, no km wasted, and no runes stop regening. The ~30k dmg boost from waiting is a 100% gain. Wait as long as you can without wasted resources.
With KM, but not enough runes to obliterate:
Every second you wait, there is a chance that KM is overridden. So how long is it worth waiting before the override renders waiting not worth it?
KM has 6 ppm, and fully raid buffed most dks have around 50% haste. That's 9 actual ppm, or .15 pps. Therefore, every 6.666 seconds, on average, you will get one KM proc. If FS gets an x boost of damage from KM, obliterate gets about 2x. Therefore, with average luck, waiting the full 6.666 seconds makes you break even, whereas waiting less becomes a bigger gain the less time you wait, assuming you get KM to actually proc. This is also assuming you continue to not allow the other resources to overcap.
Priority 1: Not wasting resources
Similar to 2-hand, with the exception that you want to spend runes on howling blast instead of obliterate most of the time, and KM works differently.
It's still very important to avoid capping on any of the 5 resorces, for a more in-depth look read through the 2hand section.
Killing Machine procs:
KM still procs on melee, but is an entirely different story for DW. Unlike 2hand, where oblit hits about twice as hard as FS, FS *only* hits about 15% harder for DW, and therefore has different rules. Therefore, combined with the fact that DW procs are all but doubled with 2 weapons, when you have KM you want to get rid of it as fast as possible, if you don't have the runic for a FS, you want to use it up on an obliterate.
Mastering unholy runes
In the case of duel wield, even before stacking mastery, 2 howling blasts hit harder than an obliterate, and as such howling blast takes a far more prominent spot in our priority. Unfortunately, while both frost and death runes can be used for said howling blast, unholy runes cannot, often leaving us with 2 ugly green runes to get rid of before we can go back to spamming HB and FS.
There are three ways to get rid of unholy runes, let's see which is the best.
Death and decay: 50k dmg
Plague strike: 24k dmg + 100k (full duration diseases, 10k per 3 seconds)
Obliterate: 80k dmg +45% chance for free HB = 115k dmg
HB: 79k dmg
Obviously your best bet is double HBs whenever possible, but let's say you have 1 unholy and 1 frost/death, what should you do?
In order from best to worst:
Using plague strike and HB gives 203k dmg IF diseases are about to fall off
Using plague leech, plague strike and 2 howling blasts gives 182k dmg
Using DnD and HB gives 129k dmg
Using obliterate gives 115k dmg
Using plague strike and HB gives 103k dmg
Ignoring the unholy rune and just spending the frost/death on HB gives 79k dmg
While it's true you can use plague strike and howling blast at half disease duration and count the 50% refresh as 50k dmg, it prolongs the ability to fully refresh diseases via the top option for 15 seconds, and is therefore not a damage gain. If blood plague is not just about to fall off, count it as if it isn't refreshing at all.
So, if diseases are about to fall off, plauge leech is off cd, or dnd is off cd, use the above options to remove unholy runes, otherwise just use obliterate if you have to.
Mastering blood tap
Blood tap micro-management is very interesting, and useful. Whenever you use frost strike, you add 2 blood charges to a bank, and whenever you use blood tap, you trade 5 charges for a death rune. The first step to correct micro-management is to avoid allowing the blood charges to be wasted via over-capping, or letting the charges fall off. This simply means don't use FS when you have 11 or 12 charges, and don't go 30 full seconds without using FS or blood tap. After doing that, it's ideal to find the oppertune moment to use those built up blood taps. I've found two cases where it's worth not using them as soon as you have a free GCD.
Pairing blood taps with unholy runes, when DnD/PL are on CD, and diseases are not about to fall off, allows for very easy management of said unholy runes, and helps to avoid the case of overzealously spamming HB and not having the second rune to pair with the unholy rune.
Saving blood taps for times with your trinket procs are up, or lust, or potions, or burn phases, leads to a damage increase. If you are GCD capped when you have procs up, you're doing it right.
Encounter specific tips, tricks, talents, and glyphs
Feel free to add in more in the comments, and I will update the OP. Please test any that are untested, should you get the opportunity, and report your findings back.
The Stone Guard
Having Death's advance makes you less susceptible to the petrification slow than other players, and activating it makes you move at 100%, regardless of how many stacks of petrification are out.
AMZ can absorb an overload quite nicely for the raid.
AMS absorbs the boatload of azure mines, and makes you immune to the root.
Feng the Accursed
Gorefiend's grasp is fantastic for stacking the adds during the shield phase, and moving them further away from the shield. I prefer using it as soon as the adds spawn, standing 10 yards away, and targeting myself.
You can also single deathgrip any adds about to run into the shield.
Chilblains can be used to slow the shield phase adds.
Gara'jal the Spiritbinder
Remorseless winter allows you AoE stun the adds in the spirit realm.
Death pact is very good for burst healing when you have a voodoo doll, or are in the spirit realm.
Glyph of dark succor constantly procs, giving not only exceptional, free self heals, but death strike also does decent damage for free.
Using army after the boss is pulled, resetting the boss by running him out of his room, and pulling shortly after he re-spawns allows you to have double the army ghouls up for a short time. Unlikey to be put up with during progression, but might be fun for parsing. (untested)
The Spirit Kings
Gore-fiend's grasp allows you to stack the raid during maddening shout, allowing the group to avoid stacking beforehand.
You can use dark simulacrum on raid members during maddening shout. I suggest asking for a mages armor (1 min of 5% crit or 3k mastery), a warlock's darksoul, or a warlock's doom-guard. Fun fact, if you have symbiosis from a druid, and manage to Dsim wild mushroom: detonate, you can detonate your disease mushroom!
Glyph of Dsim makes the above even better.
Avoid using howling blast during maddening shout.
Avoid casting soul reaper when the shields are about to go up, as it will blow the shield up.
Icebound fortitude will remove the pinning arrow in the rare event you get hit.
Glyph of pestilence allows your diseases, when spread from the boss, to hit all 6 orbs during p2.
AMZ is a nice raid CD during the final burn phase.
By pooling runes, and KM procs, death knights are one of the best classes for killing orbs.
AMZ should be able to absorb total annihilation outside for the entire group (untested)
Will of the Emperor
Gorefiend's grasp can group the rages together quite nicely.
AMS allows you to absorb a titan spark (untested)
Line up your cooldowns with titan gas (irrelevant in heroic)
Chilblains works on both the rages and the courages, and can slow a courage before it puts it's shield up, if timed right.
Heart of Fear:
Imperial Vizier Zor'lok
Dark simulacrum works on raid members affected by convert.
AMS prevents the damage and silence from the gas in the center of the room.
Blade Lord Ta'yak
Purgatory cuts down on gibs from unseen strike.
Glyph of shifting presences, makes switching to blood presence during the tornado run at 20%, and 10% even easier to survive, and all but removes the already slight runic loss.
Using HB on the body hits front legs (untested)
Wind Lord Mel'jarak
Both desecrated ground and icebound fortitude break the amber on you. Simply run 5 feet outside the melee, pop one, and run back in.
AMS may prevent the application of amber (untested)
Dark simulacrum can be used on amber constructs shortly before they break out, and remains with the player after they are broken out, allowing you to copy spells. I once again suggest going for mage armor, or warlock cooldowns, but it's normally worth trying your luck with whomever becomes a construct.
Glyph of Dsim makes the above better.
AMZ is great for absorbing an entire explosion, from either a free add or the monstrosity.
Grand Empress Shek'zeer
Chilblains slows the windblades.
Remorseless winter is a very effective stun to prolong the life of the trap, after you've trapped a reaver.
Lichborne breaks both fears in the final phase, and AMS prevents the application of fear (untested)
Terrace of the Endless Spring:
Protectors of the Endless
The lightning stun can be broken with IBF and Desecrated ground.
The adds during the day can be both deathgripped and gorefiend's grasped.
AMS prevents the fear if used preemptively, and lichborne and desecrated ground both break it.
During get away, glyph of pillar of frost's root prevents you from being sent back, and the lack of overall movement during the fight makes the root nearly unnoticeable when get away is not being cast.
The adds can be moved away from the tanks, possibly saving them, with both gorefiend's grasp and death grip. Glyph death grip to not taunt though, or the adds will kill you instead.
Sha of Fear
Lichborne/desecrated ground breaks both the breath of fear and the dread spray on the platform, while ams prevents them from being applied.
Orbs on the platform seem to regenerate your runes, not runic (not fully tested)
Have spent more than a few hours on a simcraft profile illustrating the benefits of waiting, and I've been so far unable to prove a gain. While increasing the critical rate of obliterate is easy, the sim, being priority based, insists on wasting resorces, leading to a net loss. If anyone comes up with a functioning sim I'll gladly add it in, but for the time being I'm abandoning the search for the right profile. Waiting continues to provide a gain both logically, and in practice, and the best way to prove that is with more feedback.
All damage data taken from BiS sim, adjust your own math for your own numbers.
2hand, mastering soul reaper
DW, getting the most out of cleaving
Both, CD management and trinket procs
Both, swing timers and you
10/20/12: Added mastering GCDs section
10/22/12: Added mastering symbiosis section
11/06/12: Added how long is it worth waiting section
11/14/12: Updated note and fixed minor spelling and grammer issues throughout the post
11/20/12: Added encounter specific tips, tricks, talents, and glyphs and fixed more spelling errors
11/22/12: Touched up encounter specific tips, tricks, talents, and glyphs, and removed suggested spec due to controversy/started working on DW section
11/24/12: Added DW section, including priority 1: not wasting resources, mastering unholy runes, and mastering blood tap, as well as planned sections