Frenzied Wombat
Potato del Grande
To be honest, if your problem is falling asleep and not staying asleep, just go get a script for Ambien. It will knock you out in 15 minutes flat. Just make sure you only take it when you really need to and can get a solid 7 hours sleep. Back in college, I would pretty much always take one the night before an exam as I knew I would have equations running through my brain preventing me from falling asleep.
Dealt with this a lot over the last few years, best advice I've seen in this thread so far are to avoid any form of light and don't look at the clock. It always starts for me by looking at the clock and starting to worry that I won't get enough sleep by the time I need to wake up, or I'll sleep through when I'm supposed to wake up and be late.
Melatonin did nothing for me, but my problem has always been falling asleep in the first place. Last night I got in bed at 1am and didn't fall asleep until about 6am. This was the first night in about 3 weeks I've had trouble sleeping. From about late November through mid-December I was freaking out about finals/projects due in school, once that all ended I was at ease and fell asleep pretty easily. The biggest factor I've found that helps me sleep is when I just don't give a fuck about what I have going on the next day. Sounds weird, but it puts me at ease and I fall asleep easily.