There is, sadly, no magic pill to combat this malaise that afflicts travellers when they cross time zones. After a 20-hour flight, with a stopover, to the US, you might find yourself arriving (in terms of time) only 10 hours after you took off. The human body isn't used to changing so many time zones so fast and jet lag is the unfortunate result.
What is jet lag?
Many people suffer from jet lag when their regular darklight (day-night) schedule is abruptly altered. Contrary to popular perception, it has nothing really to do with travelling in a jet plane, per se. You can suffer jet lag if you work shifts that alternate between night and day, from living in a city which practises daylight saving time or, as the name suggests, by travelling fast (as on a jet plane) across time zones.
Here, you must keep in mind that it is the act of travelling across time zones (before your body can adjust to them), rather than the fact of travelling on a jet plane that plays havoc with your bio-rhythms and causes jet lag.
It is supposed to be easier travelling West (back in time) than travelling East (earlier in time) but you can't plan your trips travelling West all the time. You can, however, take some steps to make combating jet lag a lot easier.
Adjust your sleep and meal cycles to that of your destination a few days before you travel. So, if you're travelling to Sydney, Australia, start hitting the sack a couple of hours earlier.
Get lots of sleep before you travel to compensate for any sleep you will lose during travel. The entire process of travelling is quite strenuous, from customs and immigration and the possibility of chaotic transfers in foreign airports. If you are tired, your jet lag could be worse.
Avoid alcohol and caffeine products during and after the trip. These are both diuretics and dehydrate you and can again aggravate your jet lag.
Ditto for salt; cut down your salt intake. This is particularly pertinent because of the fact that the ambient air within an airplane tends to be very dry and dehydrates you even more. Drink lots of water and juices.
Move around in the flight, this prevents the formation of potentially deadly blood clots in your veins (this malaise is known as deep vein thrombosis or DVT).
Don't dress up for the flight; wear comfortable easy fitting clothes. Dressy clothes can keep you from relaxing on-board.
Try and time your flight in such a way that you arrive at your destination in daylight hours. Bright light, particularly sunlight, is the best thing to help you combat jet lag. Avoid late-night "Red Eye" flights when crossing time zones.
Whenever you arrive at your destination, start adjusting to the time zone promptly by changing your meal and sleep cycles.