Hm... I wish I knew but sometimes I still have allot of trouble sleeping D:

I have some ideas. I suffer from on-again off-again sleeping troubles myself. You can try conditioning yourself to associate bed with sleep by not doing ANYTHING else on, in or near your bed (or even in your bedroom if possible) except sleeping. If you are reading, or studying or whatever, do it somewhere else. Eventually your mind will connect the act of laying on the bed with sleep and will know to automatically shut off when you do so.

I find it helpful to use another "marker" besides laying down. I keep the same routine before sleeping all the time. Pick something soothing. A cup of tea? Brushing your hair (if it's long), listening to a very specific playlist on an mp3 player that you use only for that purpose. I find the music helps me most. I put only 4 or 5 songs on, something soft and quiet. I play them on repeat with the volume really low and I make sure those songs aren't on my other playlists. I can even use that playlist to help me sleep on the greyhound bus now.

I hope something in here is helpful to you. Come talk to me some more, I have other ideas too.

I know it's late on this, but try melatonin for a sleeping aid. It's an herbal suppliment, not a narcotic.