Hydrate like a boss. Get ample sleep. Try to exercise when you can - even if it's just getting in a brisk walk in the morning. Pace yourself on the booze, both inflight and if you're going to client/colleague dinners. If you're not a big drinker, nurse a glass of red wine all night long... If it's a 3-4 night trip where I control the agenda, I schedule at least one early night without a big dinner. I use that night to exercise hard (a long run if possible) and stay 100% alcohol-free.

I don't buy into all of the hand sanitizing products, but I do wash hands often and try to minimize unnecessary contact with other people or things that other people touch regularly.