During the long flight employees offer you many kinds of foods that you can eat, so choose your favorite one and have a good meal, but besides this avoid of eating some kind of foods that produce gas such as onions, radish, beans, etc and some beverages such as beer, drinks with gas are might be the cause of stomach.

Prepare the small bag holds your useful belongings /your personal useful things/ such as all toiletries, medication or at least a toothbrush and toothpaste etc can be used during the long flight because your luggage won't be with you while you are in the airplane passenger cabin. So don't forget keeping your important little items in that small bag. But napkins, paper cups and stuff like that are available in the toilets.

Bring a good book or something to read. Airlines keep you busy with food and entertained with movies and other films, but there might be lot of time to get bored. So read your favorite book or something interesting and enjoy your flight.