What do you do to focus?

Usually when I need to study, my mind is able to get right to it and I get what I need to get done accomplished. I don’t know what it is right now, but I can’t seem to keep a steady thought. I’ll be reading through my notes and then next thing you know I’m starring off into space or checking my email. I’ve tried listening to music, not listening to music, going for a walk around the library, getting a drink of water, smacking myself in the head, etc. all methods have proved ineffective.

What are some of the things you guys do to stay focused (other than calling it quits and coming back later)?