Have you tried any of the relaxation tracks? I've used them when trying to sleep while jet-lagged, and actually found they work really rather well. Whack one on an ipod (and preferably in a cabin where you can do the lie flat thing so you can get properly relaxed!). Alternatively, if you've ever done yoga or tai chi, trying some of the post session relaxation techniques can work, but it can be difficult to get into that mind set on a plane.

The best tracks I've ever come across came in the US Crown Plazas as part of some special room ambience they were trying to create/market - unfortunately, I didn't buy one at the time, and they were going to charge me $60 to ship to the UK when I wanted to buy one later...