As a result of all sorts of things going on in my life right now, I've been unable to get my brain to shut off at night. And that's preventing me from getting to sleep.
I've got a few tricks that I typically use in these situations, the first of which is a lifestyle choice.
1) TV/computer off by 8:30. This lets my brain calm down. I clearly don't have any scientific evidence behind this, but I know it works. There's so much noise on TV and the Internet -- when I cut it off early, it gives me a chance to wind down. When I'm not consistent with this, it's usually a sign that I'm over-stressed (either mentally or physically or both). Last night I powered down at 8:00. Tonight I'm planning on not powering up at all.
2) When I'm laying in bed, if I can't get to sleep, I start thinking about relaxing my body, starting from my toes and working my way up. I usually get bored around my calves and nod off.
3) If I still can't get to sleep after trying #2, I use this trick that I learned from this guy (while not even remotely endurance related, this site is impressive if only as a demonstration of the human ability to change its situation). With your eyes closed, look side to side as fast as you can for as long as you can. It's pretty hard and for whatever reason, really knocks me out. The only drawback is that if I need to do it for a long time, my eyes are tired in the morning.
4) Get completely hammered. Unfortunately, this doesn't help with the quality of sleep, but at least it gets my brain to stop working.