I have to take a 13-hour flight to China. Drink plenty of it. Take a couple of 1-liter bottles of water with you and keep yourself well hydrated throughout the flight. I would avoid caffeine, and alcohol as well.
Food is harder to digest at high altitudes.
Sugar isn't helpful either, because it sends your blood sugar on a roller-coaster ride. I have an acquaintance who travels regularly to China and Japan. As soon as the plane takes off, change out of your shoes and into a pair of thin slippers. Bring along some juicy fruit to complement your water supply. Try earplugs to shut out the noise.