If you've got a laptop/portable dvd player, it's a must albeit a pain to carry. We brought lots of snacks, crayons, a notebook and a couple board books. Prior to the flight, we went out and bought a small toy for each of them, wrapped it and let them know that they each had a "special something" that they could open as long as they were good. It was nice and they had something new to fool around with. Lastly, we made sure we had some lollipops/gum/juice for landing and take off (helps with popping their ears).
Best of luck!