Do you exercise a lot or are you sedentary during the day? I find if you use a lot of energy, you sleep better. If you're already doing that obviously it won't help.
I agree with setting a usual bedtime. Also try and have a routine that you do whenever you get ready for bed. Read for 40 minutes, or play on a handheld/phone, watch a specific show. Take a nice warm shower. Drink something warm that isn't caffeinated.
Don't stay up on the computer though... I can go all night glued to a computer screen.
Then, when you're in bed, don't TRY to fall asleep... I find that, for met at least, doing that just makes my brain race. Just try and be comfortable, and do something that doesn't stimulate you too much but occupies your attention.
For me, I have played my DS so regularly in bed before sleeping while watching the Daily Show/Colbert Report (they air at midnight/12:30 where I am), that I'm nodding off automatically out of habit. Specially if it's a DS game that's turn based (e.g. Pokemon > Gardening Mama, for example

). I actually bonked myself in the face once because I dozed off while holding my DS up.
I get something like this a few weeks a year where I can't sleep well and am just tired all the damned time. Being exhausted sucks, so I hope yours goes away soon.