Noise-canceling headphones. Eye mask (some planes might provide one, but get a nice one that fits you). A good mask (planes can be very low-humidity so even if you aren’t worried about anything else, this can help keep you more comfortable).
Wear loose-fitting clothing (except compression socks if that’s something you want to do) or otherwise comfy clothing.
Other than that, get up and stretch every couple of hours if you can and are not sleeping.
I have flown from NY to Tokyo multiple times (which is like 15 hours depending upon weather).
I got a Manta Sleep mask on the recommendation of a friend last year and it is so, so good. I take it everywhere with me now, and use it on any flight where I need to get some rest. That and good earbuds make a huge difference if you’re a light sleeper like me.
Which type of Manta did you go with. They apparently have quite a few variants, and I might invest in one for my next flight.
I have the Pro version because I’m a side sleeper and their site said it was great for that. They were right! Other sleep masks I tried would come off by morning but this thing stays put. It’s really comfy.
I sound like an ad but I usually wake up as soon as sunlight creeps into the room, and this thing helps me get an extra 1-2 hours during summer, which is huge. I really love it.