In general the best advice is be prepared! No one has mentioned Sudoku. What I do is go to the website web sudoku and print off a bunch of sudokus. I then put them on a clip board so I have a nice easy way to complete it. 1 sudoku evil will on average take me about 45 mins.. (I'm slow.) The nice thing is, the time flies by and you don't even notice it. Do 4 of these, and you've knocked 3 hours off your trip.
Echoing what Mark said, I dress super comfortably, and warm as well. I basically wear a nice comfy sweatsuit.. (It's as fashionable as a sweatsuit can be..) I actually always wear the same thing, and have it planned in advance.
If you can put like a whole TV series on a mobile device, that would be advantageous too. Something like an HBO series or something. One season of some shows can be like 13+ hours.
I actually like to exercise a lot before an upcoming flight so I can increase my chances of sleeping during the flight.
Study the language of the country you're going to if it's different.
Luck! Get an empty seat next to you, or someone frail so you don't have to fight over the arm rest the whole time!