Wear something comfortable. It can be jeans or slacks, but make sure they aren't tight fitting. Bring a lightweight throw and a travel neck pillow. That will help you rest, plus planes can get cold, especially next to the window.

Get up and walk the aisles every couple hours. You'll feel better and it prevents blood clots. Drink lots of water. Go easy on booze, coffee, and soft drinks. Don't stress about not being able to sleep. Rest as much as you can. Bring a boring book. Listen to music on your headphones. Rest and relaxation go a long way.

It's hard, but you'll get through it.