If you’re desperate, a benadryl (or house brand) will help you fall asleep. (You must be ready to sleep, though—it’s somewhat easy to shrug off the sleepiness it causes if you’re up and about.)
I’ve heard good things about melatonin, but it can take a while before it starts working correctly.
Warm milk? (It works for some people.) So does chamomile tea, or really, any non-caffeinated warm beverage.
A shower before bed will help you sleep, too—something about body temperature, I’m not sure.
A routine will also help you sleep better, but like with the melatonin—you need to do it for a week or so before it starts paying dividends.
If you’re like @rooeytoo and have a “loop” going in your head, try writing everything in your head down on a piece of paper. It may or may not help.
Something that works really well for me: tell yourself a story. It can be about anything, really, just start telling yourself a story—make it involved and interesting, and before you know it, you’ll be falling asleep.
You can also try just clearing your head of everything—the best way I’ve found to do this is to have a set number (i.e. 50) and count backwards from that, breathing in and out for each number. This forces your brain to focus on nothing but the counting, and can often “pave the way” to sleepy-time. (Try to do this when you’re in a comfortable position: it works best when you’re not moving around too much.)
Good luck!
@uberbatman: ????