I usually find I cant sleep because I have something on my mind. Whether conciously or not I am worrying about something, planning something etc. I find the best way to switch off my mind is to listen to talk radio, an audio book or something, just in the background so you can half concentrate on that as you drift off.
I also find the worst thing when you cant sleep is getting stressed about it, I wake up in the night, then get annoyed as I watch the clock spin nearer to the alarm and the more annoyed I get, the less chance I have to sleep. Maybe have a bath pre bed, put on the radio and hopefully drift off. xx