Have you tried Melatonin? It's a hormone which, among other things, regulates our sleep cycles. I've found that taking a supplement of this (3-6mg or so) significantly helps me drift off. It won't knock you out like most sleep aids, it just makes nodding off easier. It can mean the difference between falling asleep in 10 mins or 4 hours for me. I don't experience insomnia as bad as I used to, so I only take it once or twice a week but there was a period when I was using it every day and noticed no tolerance buildup at all. Unfortunately this isn't available OTC (don't get the herbal remedies.. they contain very little melatonin) and I haven't come across a pharmacy that stocks it, so you'll need to get a prescription then send it to a compounding chemist who will mail it to you. I'd give you the name of the compounding chemist I order through, but I think this may be against the forum guidelines (could a mod PM me if this is okay?).
Your doctor shouldn't have a problem prescribing Melatonin as long as he's familiar with it. If he isn't, you may need to do some research to convince him, as it isn't listed in MIMS. It has a very low potential for abuse as there is nothing remotely resembling a high while under its influence and the only withdrawl I've ever experienced through not taking it is the return of the symptoms I was using it to treat.