Here's my advice on long-haul flights:

- Drink plenty of water. Bring your own bottles of water so that you will always have plenty. Even if, and especially if, you drink alcohol before/during/after your meal, follow it up with water. This is always good advice with alcohol, and especially so in the high altitude, low humidity environment of the aircraft cabin.

- Get out of your seat and stretch your legs, even when you don't have to go to the bathroom. Doing this every hour or so (when awake) will make a big difference. Request an aisle seat so that you can get up without disturbing your neighbor. On long flights, I'll take at least one "seat break" where I stand in the galley or near one of the doors for 15 minutes or so - it helps a lot.

I think a lot depends on whether or not you generally have problems with jetlag. I tend to not have problems in the outbound direction, as I'm excited about getting to whereever I'm going. The only issue I ever seem to have is on the return segment. As such, I don't worry about adjusting sleep or eating schedules in advance. But, if someone has a known issue with jetlag such that it will interfere with their trip in a big way, perhaps some of the more involved remedies that others have suggested would be in order.