I currently travel something like 250K miles per year. Here are a few tips I’ve learned from fellow travelers:

1. Plan ahead with clothing. This will be a life saver! Always have layers of loose fitting clothing. The idea is that if it's loose you'll be more comfortable and avoid having to use the toilet every 30 minutes. In addition the layers allow you to control how the ambient temperature of the flight affects you. Take off what you need or put on what you need when you need it. Jogging pants, T-shirt (long sleeved), a hoody and slip on sneakers are ideal
2. Have a fully charged tablet with mindless games and plenty of media to consume. You never know what sort of entertainment system the airline will have until you board the flight. In addition the reliability of the systems are in question. On average last year, 1 out of 5 flights I was on there was an issue with the entertainment system…
3. Jetlag is manageable. What seems to work with varying degres of success: I try to calculate the time of the first breakfast I’ll have at my destination and I stop eating anything about 18 hours before that. The idea is to hard-reset the digestive system to the new time via hacking the signals between the gut and the brain to make it think you just woke up after 18 hours for breakfast. This usually works for me (going east or west) and it shortens the effects of jetlag by at least 2–3 days.
4. Hydrate and avoid alcohol. I know it's soooo much easier to get through the 8–12-hour flight by tying one or three on… But the effects of those drinks last for days after the flight is over. In addition drink water, lots of it.
5. Sleep. Take every opportunity to get a few Zs, they’re precious on the ground and even more in the air.
6. Noise-cancelling headphones are bliss. 300 people in a turbine-propelled aluminum tube for 12 hours is extremely noisy. That noise causes a lot of low level stress over a long period of time. One of the ways to counter this is by investing (yes this is an investment into your sanity) into noise-cancelling headphones/earbuds/whatever. You‘ll thank yourself later.
7. Smile, nod and understand. Most of the issues we have with long flights are the people around us. The crying baby three rows back, the guy who kicks your seat with each shift of his body, the teen who's trying to get the movie on the screen to play by pounding the screen attached to your seat… All of these are things that are annoying and completely out of your control. You need to be at peace with them… I know it's hard (lord I know…) but it will save you bucketloads of stress to let these things pass.
8. Books. Bring a few, it helps with passing the time and of you bring one that is on the boring side, it will help with item #5, sleep.
9. Deck of cards. If you’re traveling with a mate, a deck of cards can help pass the time. Who knows, maybe your seat mates on the other side want to play to pass the time too.
10. Compression socks aren’t just for old people. I have to admit, I thought this was bunk until I tried it. Compression socks apply pressure to the feet and calves and prevent them from feeling like lead on the end of your legs due to sitting for extended periods of time. I find that wearing them significantly improves my ability to physically function at a normal level directly after a 12-hour flight.
11. Always have these four things handy: nose spray, lip balm, hand lotion, eye drops.

I hope this helps!