Though this article is written primarily for Canon DSLRs, Nikons are also able to utilize back button focusing. I'm told there's some sort of ritual where you must light a 16:9 rectangle of candles, expose your sensor, sprinkle holy water on the hot shoe, then chant "Incantus focalis backeous buttonous" in ISO 666, or something to that effect. Or just follow this tutorial here.
What is Back Button Focusing (BBF)?
If you're a beginner or even an amateur hobbyist, back button focusing may be something you've never heard of. What is it? It's a method of shooting where auto-focus has a dedicated button. This means your shutter is also just a 1-stage shutter button now.
Normally, as most of you know (oh, I hope you know this), you depress the shutter button halfway, which triggers the auto-focus. The second stage triggers the shutter, which gives you the shot. With back button focusing, if you depress the shutter button halfway, nothing will happen. To trigger auto-focus, you must press a separate button (depending on your model).
Why would this help me?
Back button focusing is especially advantageous when combined with manual focus points and an Stepping (STM) or Ultrasonic (USM) motor, as these have constant manual override. If you've ever shot with one of these "constant manual override" lenses, you'd know that the functionality is so-so. If you focus manually then press the shutter button, before you know it, your focus has been blown by the auto-focus kicking in halfway through the shutter press.
With back button focusing (with a STM or USM lens), you can:
- Manual focus all the time while leaving the lens in AF
- Pick your focus point (if you use manual focus points), then recompose the shot without refocusing
- Make those really cool noises that the STM motors make because you don't have to half depress the shutter button to play with the manual override.
See that AF/MF switch over there? Fuck that switch. (Source)
Not to mention the fact that you're able to do a sharp shot and bokeh all without having to flip a switch on the lens so that you can have control.
So How Do I Enable BBF?
If you've got a EOS Rebel (T1i, T2i, T3i, T4i, T5i), the menu you're looking for is under the third wrench icon, under Custom Functions.
The function you'd like to change is:
C.FnIV: Operation/Others 
Shutter/AE Lock Button
Change the setting to 3: AE/AF, no AE lock.
Ok, Now Tell Me How to do it on My 5DmkII.
For the full framed cameras (I'm specifically mentioning the 5D because that's where I'm taking instructions from), find your custom functions menu and select:
C.FnIV: Operation/Others  (Most lefthand Setting)
Change from 0: Metering + AF Start to 3: AE Lock/Metering + AF Start
The menu looks something like this. It's hard to get good pictures when the person you know who has a 5D Mark II doesn't have another camera around her to take a picture of the screen. This should be readable though! TheGirlfriend has offered to transcribe the text if you need it!
I've done it. Now you've forgotten to tell me how to focus.
Shit, my bad. For the T2i, here's a picture for your convenience:
Press that button. Remember that pressing the button next to it and scrolling the wheel will allow you to navigate your manual focus points. Also, if you review your pictures, half depress the shutter to snap back to snapping mode (yeah that one was a bad one), or you'll find yourself frustrated that its not focusing and that when you go to preview all your pictures are really zoomed out...
For the 5DmkII:
(Source) See that AF-ON button up there? That's your new best friend.
5D Mark II owners who have followed my instructions for BBF should find that the AF-ON button should auto-focus now.