I think a few other moms mentioned this, but it's worth emphasizing: give the kids only one thing at a time. This works at home too - my kids play far longer with a single toy than if they have five or six toys out at once. The same works for travel.
When my dad took us on long trips, he would mark a map with points at which he would get out a new item. We loved watching the mile markers and counting down until the next thing. For a plane ride, you might try a timer - one toy every 45 minutes or something. You could use a stopwatch to count up instead of a timer, so there's no noise to bother other passengers (there are also flashing and vibrating timers, but that's more expense).
You might also want to bring along a small book of kids photos - my kids love looking at pictures of themselves, and will look at them over and over.
You could also try an airplane game - either a bingo-type game (window, seat, tray, cup, person wearing red, etc.) or a "drinking game" type (without the drinking!) where you get a point for each time the captain comes over the loudspeaker, each time the flight attendant walks down the aisle, etc. Have the kids suggest other ways to get points.
And one more tip - bring minimal stuff for you to do yourself. I always have a worse time with my kids if I bring a book of my own - I'm always trying to get them to sit still so I can read "just one page" and it makes me frustrated and snippy with them. If I didn't have the lure of my adult activity, I wouldn't be as impatient.