If you have to sit in an aircraft for more than 10h there's a few things that are absolutely essential to not feel completely dead on arrival. Some of these are very obvious, some of them took me a few flights to figure out and some of them are just opinionated and very much debatable.
Noise cancelling headphones (Non negotiable)
Even if you don't listen to music and just wear them with the noise cancelling mode activated these are life savers and if you did it once you won't be able to go back or even understand how people can fly without a pair of these. I usually listen to podcasts or random noises through apps like Rain Rain.
My recommendation: Bose QuietComfort 35
One of my favorite recent discoveries was Craig Mod's new podcast called SW945 which consists of daily field recordings from his walk in Japan. You can read more about it in "The Glorious, Almost-Disconnected Boredom of My Walk in Japan" published on Wired. I listend to all episodes on my flight to Kuala Lumpur and was enjoying it so much that I immediately sent an email to him sharing the experience after landing.
My inner fanboy got a bit excited when I saw that this somehow made it into the Wired piece:
My hope was that others could “listen along” to the walk. Someone emailed and said that, on a recent long-haul flight, they had put in noise-canceling headphones, covered their head with a blanket, and listened to the walk for five hours. This made me unreasonably happy.
This describes my way of traveling pretty accurately, I try to sleep as much as I can, listen to relaxing background noises when I'm not sleeping and use the provided blanket to block out all the distractions as good as possible.
Food & Drinks
Nothing with caffeine as my feet get nervous if I drink too much of that and then have to sit for a long time. Lots of water, sometimes beer to help me sleep. Be very selective with the food that gets served, as everything will be thrown away anyway eat the part that is easily digestible and leave the rest. Bring cereal bars and proper sandwiches instead (Except on AirAsia, food is excellent). Don't bring food that smells and bothers people.
Bring your own empty bottle and ask them to fill it up with water when they do the first service. That way you don't have to pay attention for them to come by, have to find a place for the plastic cups and wait for the next cycle of them picking up the trash again. Makes everything easier and you can drink more while not producing more unnecessary trash. The air in planes is very dry.
I was always a big fan of window seats but if I can't get a bulkhead seat or one where I can easily leave I'll stick with the aisle seats now if the flight is longer than 3 hours. That way you can get up as often as you want, walk around, get more water if you need to and use the bathroom. Do your seat research with SeatGuru and pay for a good seat. It's usually well worth it and you don't have to worry about being late for online check-in.
Don't use them and especially don't put important things like boarding passes, phones, passports in there. It's a recipe for disaster apart from them being one of the filthiest spots on a plane. Also don't use the bathroom without shoes, it's probably more disgusting than you think and planes are not cleaned as often as you might think.
(Don't) Recline your seat
Nothing gets my blood boiling more than people who recline all the way in Economy class. Once everyone is sleeping that's probably fine, any other time not so much. There's a very good article on the subject from the Points Guy. If you want a flat bed you should probably pay a bit more.
While it may seem very cool to sit around at the gate until the very last minute it's actually just making life harder for yourself. The overhead bins for cabin luggage are usually filled up pretty quickly and people coming in last have to find some spot for luggage which may not be close to their actual row or worse, a few rows behind them. If you have a tight connection and then have to go against the stream of disembarking passengers with huge suitcase to get your luggage first you are going to have a bad time.
Travel light, if possible
Traveling with just your cabin sized luggage will make your life so much easier and less stressful. Queue to check-in your luggage? Nope. Stare at the slowly moving luggage belt waiting for it to spit our your luggage? Nope. Worrying about what you are going to wear if it doesn't spit it out? Nope. On top of that you are also not the person that needs 4 seats on the airport train for their luggage. Wins all around. Roll your clothes tightly, don't bring things you can easily buy at the destination (Toothpaste and other things that you'd have to separate out at the airport security).