Alternate a 'feast day' (high-protein breakfast and lunch, high-carbohydrate dinner) with a 'fast day' (soup and salad) for 3 days before departure; during those days don't drink any liquids containing caffeine except between 3 and 5pm. On the flight day eat sparingly and drink caffeine-laden liquids only in the morning if you're flying west, and between 6 and 9pm if you're headed east.
Preadjust your bedtime. If flying east, go to sleep an hour earlier each day for each time zone you're crossing; if flying west, move your bedtime later in the same fashion.
When flying east over several time zones, try to take a morning flight instead of an overnight one. On arrival your 'body time' will be set at mid afternoon; local time will be early evening. Eat dinner and go to bed late by local time. The next morning you'll feel rested.
Flying west is easier. In fact, you may avoid jet lag altogether if you take a late flight, stay awake during the trip, then go to bed as soon as you reach your destination.
Drink lots of fluids before and during the flight, but avoid alcohol.
Go outside into sunlight as soon as possible after arriving. Take a brisk walk or jog; moderate exercise may help your body adjust faster.