A few tips from a frequent flyer:
1. Sesame oil into your nose on the evening before the flight and on the morning of the flight. Just put a bit of oil onto a spoon and rub it into your nose, sniff a bit in. This will prevent any problems with the air on the plane, especially if you sleep a lot, get a scarf ofer your mouth as well.
2. Running shirts / Running Sweater on the plane. The kind that keeps the warmth but lets out the sweat. You'll sweat when you sleep, and there's nothing worse than sitting in a plane with the air condition at full power in a wet shirt.
3. In-Ear-Headphones. You can listen to music or use them against the noise in general.
4. Protein bars, or chocolade or any other snack is fine, but depending on the airline you will get a lot of stuff for free on board.
5. Slippers. Don't wear your shoes all the time, get some comfortable slippers or thick socks into your handbaggage.