1. Book the vegetarian meal. Eat it quickly when it comes (usually first as a special delivery) and put the tray under the seat in front of you. When the regular service comes through get a 2nd meal.
2. DO NOT save one of the small liquor bottles and bring it home with you. DO NOT fill it with gin and bring it on your next flight. Ask for a tonic water then absolutely do not pour said gin into the glass. Because it’s illegal. So don’t.
3. If they serve you a glass of something, ask for the whole can. 9/10 they’ll give it to you. Works well with #2 above.
4. If your seatmates are asleep and you are on the window and need to get out. Stand up on your seat and use the armrests to step over them to the aisle. Its tricky but its also fun. (NOTE there seems to be a lot of disbelief over this one, so I will video myself doing it on an upcoming flight and report in this answer - stay tuned).
5. On a long haul, bring a cup of noodle soup. Ask for boiling water and enjoy a salty meal in between other meals.
6. If the toilets are relatively empty, go in there, sit on the closed seat and stretch your legs up the wall. Helps rebalance the blood circulation. Hang out for 10 mins like that - its one of the few places on a plane you can get your feet above your head for a while. But dont be an ass and do this at peak times for the toilets.
7. Break a long flights down into chunks - a 12 hrs flight is 4 x 3hr flights. One of those chunks should be sleeping or dozing if you cant sleep.
8. Netflix and Amazon downloads to an ipad
9. Bose Noise Cancelling headphones, or earplugs if you cant afford.
10. The headrests are adjustable. They go up and down and the wings fold out to rest your neck against. Most people dont know.

Bon voyage!