Smoke a bowl, if I have it. Helps me relax enough to sleep, I'm quite the insomniac most of the time. Reading is good sometimes, but if the book is good then I have a tendency to read 5 chapters before finally bringing myself to close it...
I've tried listening to soft music, or having a slow-paced movie going, but I'm the kind of person who's mind just latches onto any stimulation that it is receiving. If I hear a movie, I'm automatically interested in following it. If I hear music, I can't help but to listen to it (yes, hearing and listening are different).
And if I'm just in a dark room, with nothing on, I sit there and think. Just think. About my day, about future days, about past days, about days that never happened, days I wish had happened... Hell I'll remember some completely random event from 4 years ago, and replay it in my mind, evaluating what I said, what the other people said, and how I could have changed the outcome of the event by saying something different.
Really, I'm terrible at sleeping
Edit: To highlight this point, it's currently 4:13 am, and I have class in just under 4 hours.. yet here I am, posting on internet forums.