It may not be possible to avoid it altogether, but you can make it less of a problem with a few simple strategies.

It's important to keep well hydrated during the flight and for a few hours afterwards, so you should drink plenty of water - it's best if you start this before the flight - and don't drink alcohol. Try to move around during the flght as much as you can, and do some stretches to improve your circulation.

When you arrive at your destination, if you have been travelling east you should try to go to bed a bit earlier than usual. If you're going west, go later. In general, though, it's important to go to bed at roughly your usual time even if you feel very tired, so that your body clock can adjust more quickly. A brisk walk before bed will help too.

You can find some more advice for avoiding jet lag here.