As someone who regularly has to travel 24 hours or more to return home I can say with some certainty that no matter what you do it’s going to feel a bit like torture. I find 12 hrs relatively fine but beyond that is certainly an exercise in endurance. There are some things you can do to help.
I always take a midnight flight and try to sleep most of the way through that part of the journey. The flight from Asia to the west coast is always the most enjoyable part of the trip. It’s getting across the NA continent that seems to elicit the most pain.
Some quick tips:
Get plenty of sleep during and before the flight. For long flights try to travel at night when you naturally feel like sleeping. Whether you feel it or not traveling is exhausting. Sleep will allow your body to rejuvenate and repair itself.
Drink plenty of fluids as you can get dehydrated quickly inside a plane. If I don’t drink I can see a noticeable difference in my skin. It’s drier and I feel like crap.
Do a bit of stretching and get up and walk around whenever you can. Avoid sitting with crossed legs. This is far more convenient if you are able to snag an aisle seat. I hate stepping over people all the time.
Avoid alcohol and caffeinated drinks during the flight as they will lead to dehydration and tiredness. I always like a cup of green tea before arrival. I also try to stick to warm drinks when I can – it’s a habit I picked up from living in Taiwan. I can’t remember the exact reason but give it a try.
Eat light meals during and before the flight to avoid an upset stomach. Since you are immobile during the flight it’s hard to allow proper digestion. Despite having experienced great onboard service, I always feel much worse after eating a big meal. Even the best food is relatively unappetizing. Eat light snacks, fruits and a fresh salad instead (airlines would have to work hard at getting fruit wrong).
Take your shoes off and wear loose socks. Your feet and ankles swell immensely on long haul flights – even with exercise. This will increase your comfort (it took a couple days to remove the ‘cuts’ from my socks after one particular flight) and reduce your chances of Deep Vein Thrombosis.
I don’t recommend bulking up with books, magazines, and every device you own. You won’t use them and they only add weight to your bag. A decent airline will provide you with enough entertainment to get you through 3-4 hrs of idle time so it’s not necessary.