What are some tips in concentrating better when studying?

I find that when I study, I tend to think of other things that are not related to the topic, like another subject area or just fun activities with my girlfriend and stuff like that. I am easily distracted and my grades are really suffering because of this. I am really motivated to do better, but no matter how hard I try I cannot seem to retain that focus when I am studing. Is anyone in the collective experiencing this problem, and what are some of the ways to improve my focus when I am studing?