Close your eyes. Simple, yet true. A lot of people say they can't sleep, yet they're laying in bed with their eyes open. Closing'em is the first step and it often works. Just close your eyes and think about random things until you fall asleep.
I'm a very heavy sleeper, and usually I fall asleep within a few minutes and never wake up in the middle of the night. But every once in awhile I just can't fall asleep. Whenever that happens, I just drink warm milk, it works all the time for me. Try it.