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.
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:
I only shoot what my Mind's Eye sees