One small addition: Using focus target / having macros. I've got a macro like this: /focus [@mouseover]. Every shot is macroed like this for example:
/cast [mod:shift,@focus]Arcane Shot
One can also set a modifier as the focus modifier in settings, but keep in mind that if you do so, and still macro a shot, you must still add the [mod:shift,@focus] (or another mod), because it otherwise won't work. The focus modifier in settings only work on non-macroed abilities.
So, what's the deal with mouseover focus? Well, let's say you're supposed to nuke one target, but another is getting low and it's not super urgent that you use every single GCD on the main target. You mouseover the low target, press the /focus [@mouseover], hold shift (or your preferred modifier) and press Kill Shot. No auto shots go to waste in a target that might die before auto shot even reaches it, and you gain an increase because Kill Shot costs 0 focus and does more damage than Arcane.
(Edit: One could argue from the above example that you could just macro mouseover Arcane Shot and save 1 button because you don't need the mouseover focus. I would counter-argue that mouseover focus is very useful in other situations as well, and should not be skipped out on regardless. Additionally, it reduces the chance of error to do it the mouseover focus way if you do it well in advance. Just using mouseover, you risk the target moving at just the last second when you're pressing your ability. With focus, you can see which target is on focus before even having to cast.)
Similarly, on a fight like heroic Ragnaros (last expansion, I know) with the rolling meteors that require a SINGLE hit to get knocked, it's very useful to do this as well (just with Arcane Shot). You never drop Ragnaros as your target, and still get that one attack on the meteor. I cringed when I saw in the world first video an auto shot get wasted because both auto shot and Arcane Shot was used on the meteor.
One can even use this to use Multi-Shot through a more central target without losing the main target and potentially screwing up targeting. By never dropping the target you're supposed to nuke, you just reduce the chance of error by great amounts.
It takes a while to get used to using the mouseover focus, but once you try it out and learn it a little bit, you'll see the advantages of doing it, and wonder why the hell you didn't think of it earlier. I would say hunters are probably the class where doing this for every ability is the most useful, simply because of having a ranged auto attack.
Remember to add /cancelaura Hand of Protection in there as well, by the way (not talking to Ijanaak specifically here, but everyone).