I can't sleep on planes, even in really comfortable seats (13 hours in business class Chicago to Seoul and I barely nodded off!) so I always try to bring things to do. One trick I have is to bring along a book that I've already read and realy enjoyed the first time around. I find I enjoy reading things over again (it's amazing what you missed the first time!). I think it's perfect as you already know you'll enjoy the book, you don't have to worry that you'll discover it wasn't as good as you first thought at 30,000 feet. Plus you won't mind so much if you're interupted for something, as it's easier to put down and pick back up again.
I also agree about the ipod or mp3 player, if you have one or if you have the money to buy one before you go (and the time to download everything!). I love music on flights, since I can't sleep I find that it helps me relax at least. I lean back an close my eyes and really
listen to the songs for a change, rather than have them as background music or sing along to them in the car.
Also, most airlines have what movies they have showing on their websites, go look yours up, I find that airlines have some pretty decent movies these days, you could be surprised and they're showing something you really want to see.
And ditto that other poster, make sure you get up about once an hour and walk around, get the blood flowing again. But becareful taking off your shoes, I find that my feet swell and I have a hard time getting them back on again!
