Safe drinking water is a concern in many countries so carry your own water purifier or drink bottled water, and do not use tap water-even for teeth brushing. Don't even consider using ice cubes. Jet lag, or accommodation to a change in our body clocks, is a common problem. Forget all of the latest fads or touted cures. Avoiding dehydration and getting rest when you feel fatigued will help.
Sleep is the best antidote for fatigue, even if it is in two to three hour increments.
The air quality on airplanes is also problematic. Call your airline and find out if your flight is smoke-free. Some smoke-free airlines are Delta, Swissair and Aer Lingus. Cabin air is recirculated and very dry, so drinking a lot of nonalcoholic, decaffeinated fluids will keep you comfortable during the flight and help your system acclimate to its new time and environment. Another key to a comfortable flight is moving your legs frequently while sitting or walking in the cabin when permissible, especially on flights lasting more than four hours.