BACK BUTTON AUTOFOCUS

This is a great video tutorial about about back button focusing on a Nikon that you need to check out, I've been  using it for some time on my Canon 7D, but Canon has a advantage that it allows you to set up so both your shutter button and your back button work simultaneously . giving you the best of both worlds.
D.S.

Check Steve Perry info and video out......
Many photographers, me included, focus by half pressing on shutter release. Then we re-compose. Then we complete the press.
Photographer Steve Perry shares a great improvement to your workflow by decoupling the autofocus button from the shutter release, and relocating it to button on the back of the camera.

This decoupling allows for several great benefits:

1. You have more control over the camera. The camera does not have to re-focus every time you lift your finger from the shutter release.
2. You can instantly switch between AF-single and AF-continues by deciding how long you are pressing the AF-ON button.
3. You get great control over stationary objects and landscapes as the camera does not try to autofocus once you recompose.

http://youtu.be/FzqQskGoURE