Evian spray (not the water, but an aerosol can). This is the single best and most important item I take on any long flight: every hour or so, give your face a shpritz from the can, and you'll feel amazingly refreshed (and your skin will thank you, too). It's just6 water in a can, but it does a great job of keeping you hydrated in a very dry atmosphere.

Drink lots of water (take a 1.5 litre bottle on the plane with you), as well.