When you transmog...

[Thage]

I actually start with a color scheme or theme I want. For example, my monk's predominant color scheme is 'blue,' so she uses t14 LFR and Rayne's Scarf, along with one of the monk-quest belts that has a blue buckle, and my warrior's predominant theme is 'Argent Crusade,' so she alternates between the Insurgent Heroism-recycle set or t4 and an Argent Dawn tabard, and Righteous Armor with a Crusader's Tabard.

Weapons, I try to match up with the character's personality less so than the armor set--my monk's fist weapons are transmogged into the invisible handwraps from Peak of Serenity and her staves/polearms into the Spear of Xuen, and my warrior's weapons are usually transmogged into either the Sha axe from Terrace, or the 2h sword from Ghostlands questing.

[Deleted]

Find a piece or two in Mogit, then see if I can work my magic around it/them.

[Viradiance]

CM set, and placeholder for CM sets.

[zoomgpally]

I usually start with the shoulders and base the mog around them, I'm sure a lot of people do the same.

Usually do this, but now i start with the legendary back and base everything around that.

[Deleted]

It depends. My main usually has a set that goes well with Tabard of the Lightbringer. On my alts I go for a theme (i.e. my Monk has a Blademaster themed set). If I had to pick one key piece it would probably be shoulders for me as well.

[Arcanimus]

I start in one of 3 places: Shoulders, Helm, Weapon.
Reasoning:
These pieces are generally the most visually impactful, so its generally more effective for me to start here.

[Maxilian]

I'd start with a piece that caught my eye. It could be that I liked the shoulders of a tier set but didn't like other peices, so I'd mix and match. One set for my druid I came up with because I really liked a low level quest green set which only consisted of chest and legs, so I filled in the blanks with pieces from a variety of other sets. Other times a particular shield or weapon appealed to me and I'd make a set based around that. For my paladin I tried to make an Aldor themed set (she's Draenei) and started off with the Aldor tabard.

These, with my priest i falled in love with Mumur's mace and i made a set around it

[Leonidas]

Chest/Tabard > Legs > Shoulders > Waist > Weapons > Head > Hands > Feet > Wrists

[Omnoms]

Some times... I just mog all sorts of unrelated gear... Just to annoy people. The interesting whispers I get... well... Wow, some people take mogging very very very serious.

[Chadina]

I tend to pick a color, then a chest piece. After that I pick and choose and match the best I can, and if I find a chest piece I like better then I change it.

[angelflavor]

I think when starting off on a new character I look up tier sets. I think this is a very common way to do it. If I'm being more creative I look up random gear until I find gear based on the color I like. I also like showing off gear not available in game any more but I have.

[Deleted]

Start with chest and shoulders, after that the legs. Then the rest^^

[Kerath]

These, with my priest i falled in love with Mumur's mace and i made a set around it

This mace http://www.wowhead.com/item=29353#. ?
I love that one too, used it in a couple sets

[Felarion]

I really like mogs in theme of the race/class. For example my paladin is a Blood Elf so i try to wear armor in blood knight colors which is mostly red and black.
As an orc warrior im gonna go with savage looking armor and weapons like barbarian's theme axes or swords.

[myjackie]

mine is always based on class.

[Andyzter]

Usually to get the best result, i don't particularly pick a specific slot to xmog from, rather i just find any item that looks outrageous, and build the rest of the set from there.

[GirlsDontPlayWow]

I typically always start with an awesome pair of shoulders, or helm, and let everything flow around it.
Those two pieces are mostly what can make or break a good transmog

[Deleted]

I often just start with the most major transmog pieces, Head and shoulders, then the rest of the body and final im finding a proper weapon and/or off-hand to match with it.. Because i often find that there is (almost) a weapon for most looks

[Auxis]

I usually start with the shoulders and base the mog around them, I'm sure a lot of people do the same.

I agree. I start with shoulders, because if I start with anything else and can't find a pair of shoulders that fit well, it just throws it all off of the train-tracks. Usually it happens when I'm working with male Orcs... Given the whole size-of-shoulder-pads thing going on.

[Romeothecat]

I usually start with the shoulders and base the mog around them, I'm sure a lot of people do the same.

Same here. Shoulders are just the piece that stands out the most.