Long haul flights in Y are torture.

Salonpas (asprin patches) on your back and neck really help
Even better are lidocaine patches, but those are prescip only and really for people with more serious problems

A pillow is a must - even if your carrier gives out pillow - you need a couple of those and if the flight is full, good luck with that. Everyone has their own preferences, but look for one that can attach TO your suitcase, rather than take up space in your suitcase

Noise cancelling headphones /earplugs

Take something to read that doesn't require too much concentration

I take an ipod with my favorite podcasts, audiobooks, but a table with movies or shows downloaded will also work IF you have a power outlet

Walk around the lounge as much as possible before boarding - otherwise you just added 2 more hrs inactivity
If possible get up and walk around as much as you can (when not sleeping)
Whatever derogatory comments are made about "seat yoga" in other forums it really helps to stretch out get up, change positions etc. Perhaps not in the galley while the FA are working

Sleep deprivation is worse than jet leg, so sleep when you can. Also sleeping through the flight makes it somewhat less torturous

Absolutely don't nap when you get there - go to sleep at the time you should be going to sleep. If you think you can't stay up, take a walk, take a cold shower.

Melatonin 30 min-2 hrs before you want to sleep the night you arrive

First morning, get SOME kind of exercise close to when you wake up (I could go into the medical reasons for this and the data which are numerous if anyone is interested). Ideally 20 min in the daylight (i.e. a walk outside) this is not always possible, so even some room calisthenics near the window will work.

Long acting benzos (clonapin and diazepam etc) give better sleep quality (as long as you really have 6-8 hrs to sleep) than the short acting (ambien etc) if you really find that you have trouble sleeping at the right time.

Depending on how old you are, a hot bath upon arrival also helps. If you can find epsom salts, so much the better. When I was young, these long haul flights in Y were just boring - now they actually hurt.

Take healthy snacks - they are all cutting back on food service, and if you don't like the meal (or if it is simply inedible) that is a long time to be hungry. I am not saying that you can't have some chips if you feel like it but you are not going to feel good if that is all you have eaten for 10 hrs