I usually sleep pretty well, as in close my eyes and I am gone until morning, but I'll go through phases where I have a really hard time getting to sleep and staying to sleep.
When I first started having these problems, I'd grab a cheap bottle of wine and keep it beside my bed. I'd chug probably the equal of close to two small glasses and be able to go right to sleep. But I'd always wake up in the morning feeling like I had just come out of a dark tunnel into daylight, and needing to pee with the intensity of someone who has been holding it in for a few hours. Alcohol kills your REM cycle, so you end up sleeping long and hard, but dreamlessly and you wake up not feeling very rested. I wouldn't recommend this method until you are to that point of wanting to beat your head in because you are so tired and bored but too exhausted to get up and do anything productive.
Like DF said, cold water helps. I am sure there is a technical reason for it, but I don't feel like looking it up at the moment. Most of us drink far less water than our bodies need to function properly, so it stands to reason everything should work a little better when we drink more of the stuff. Sometimes I'lll drink a pint of cold water before getting into bed, and keep a second pint beside the bed to sip on. I am a big tea drinker, so sometimes I find a nice warm cup of herbal tea soothes me and gets my body in the mood for sleep, combine this with some reading, and I often find myself feeling like going to sleep when I was wide awake 30 minutes prior. Warm milk has the same effect for me.
Usually I can drink caffine even just a short time before bed and I have no problem sleeping, but when I hit a sleepless phase (they usually last me a week or more) I find staying as far away from caffine, no matter what the time of day, to be helpful. Perhaps this is only psychological, but since it seems to help me, I don't mind avoiding the stuff completely.
Finally, I try to avoid the computer, TV, movies, etc. when I am trying to get to sleep. After lying in bed for a few hours and still being as wide awake as when you first laid down, it can be tempting to surf the net or watch something, but I have always found this counter productive. Instead I'll read, or better yet listen to an audiobook or some old recorded lectures (you're in school, record your own or find some on the web, you'd be surprised what is out there). Sometimes these kinds of things have the ability to put you to sleep when you are wide awake, so imagine how effective they might be when you are exhausted.
Oh yeah, I've also found certain "vigorous bedtime activities" can help as well, if you have someone who doesn't mind if you pass out right after.