All you need to do on a long flight to another continent is to be comfortable, VERY comfortable. Never wear dark porous materials on a flight as the flight blankets come with lint. I made this mistake a million times. This sweater I got from H&M is very thin so I'll be layering with the black camisole.
Try to wear comfy leggings or jeggings. If you are a germ-a-phobe like me, you wouldn't want your jeans to touch the floor of the (sometimes wet!) aircraft toilet - this happens if you wear long jeans/bootcut.