Try to develop a routine for the half hour or so before you go to bed. Brush your teeth, glass of warm milk, whatever, always in the same order, and as close as possible to the same time. After a couple weeks your body learns that sleep soon follows these things, when they are done in that order at that time. It knows that you're going to go to bed soon so it begins preparing for sleep half an hour before you even climb into bed. Stick to the routine as strictly as possible. If you have a late night out, return to the routine the next night.
Like someone else mentioned, it's important not to run over what you did that day or what you may be doing tomorrow. Often you'll think of something you forgot to do, or need to do urgently, and you'll get worried about remembering to do it and will be nervous as you're trying to fall asleep. The computer is also bad
