[Otter74]

Dress presentably, but comfortably. You'll probably get a little zipper bag of things like an eye mask, thin socks, toothpaste, etc. I dislike wearing shoes on long flights and take them off as soon possible. You'll want to manage jet lag - I'd recommend reading up on that (The NYT had something good on it recently.) I flew to Malawi last October and somehow powered through 30 hours of being awake (I don't sleep well on planes), went to sleep the night of the day we arrived, slept 12-13 hours and woke up perfectly acclimated for the rest of the two weeks I was there. Went to London for a few days two weeks ago and didn't even try, I was there for so little time. You'll want to look at better advice than I can give you and consider whether your flight is an overnight or not. Getting at least some sleep will be desirable.

You won't lack for entertainment - movies, etc. I always bring a book. Noise-canceling headphones and an eye mask will help you sleep. So will an OTC sleep aid like melatonin or sleeping pills or whatever, depending on what your body takes well. Get up and stretch, at the least, every so often. I do calf, hamstring and quad stretches.

Get a seat upgrade (LOL). When I flew to London I went over in economy and managed to get an upgrade to Premium Plus (just missed business class) on the way back and boy was there a difference in comfort and that wasn't even business class.