Checked out this site www.melatonin.com and here's what they had to say about Melatonin and jet lag:

"At one time sleep experts recommended that subjects take melatonin for several days before flying to another time zone. Current research, on the other hand, suggests that melatonin taken before travel can actually worsen symtoms. Instead, you should wait to take melatonin until you arrive at your destination, and then take it a few hours before you want to go to bed for the night." (MAAH, pg. 134)
Ms. Lavert also suggests that you use light-therapy. If you want to stay awake a little longer, be sure to spend some time out in the sun upon arrival, or at least have very bright lights indoors; exercise also helps. If, when it is time to sleep, you aren't tired, sit for a while in a dark room to give your body the idea that it's time to sleep.
Other suggestions to help avert jet-lag: Avoid alcohol during the flight. Don't have much caffiene (sic), it may keep you awake at the wrong time, and try not to nap, as this tricks the body into thinking it's night.