The NHS website has some useful information about insomnia. I've linked directly to the 'Treatment' page but you might find it useful to read through the other links at the top of each page:
http://www.nhs.uk/Con.../Pages/Treatment.aspx
Listeners to BBC Radio 2 seem (during the past week) to have been giving support to the idea of eating a banana 2 hours before you go to bed.
While drugs are generally to be avoided, it might be worth visiting your doctor. When I was at college I'd not been able to sleep for over a fortnight. The doctor prescribed tranquillisers (diazepam = valium), with the suggestion that I should see if one tablet (rather than the usual two) would be enough to get me to sleep. I took just one tablet and slept for 46 hours!!! Annoyingly, I took the tablet on a Friday evening, so I missed the entire weekend! However that single tablet was enough to reset my sleep cycle and I never needed to take the rest of the tablets.
Chris