I think the others have pretty much summed up tips to deal with long haul flights. Personally I've had to fly from Singapore to Europe about 5 times in the past year! I find that it's useful to tire myself out before the flight and book a night flight if possible so that I can sleep, and I always take a window seat for long flights to have something to lean against. I always bring along a book as well, usually something I've read before that I know is really readable and I don't mind reading again, like Harry Potter. I also have this little cushion that I bring on flights as those most airlines provide are pathetically flat. Inflight entertainment is good during mealtimes.
But I must say that I'm rather lucky that I don't have problems sleeping in planes. In fact usually I'll be zonked before the plane takes off! I've fallen asleep while eating in planes, and a number of times I didn't even feel the plane touching down. A friend reckons that it has something to do with cabin pressure changes and my blood pressure....not sure how that works though!!