Honestly, it's been a trial-and-error process. I discovered a few things that work for me (YMMV, of course):
* Length of flight helps. I NEVER could sleep on international flights to Europe when I lived in NYC, because those flights were ~6-7 hours, which is, I think, the worst possible length for sleeping. By the time dinner service is done and the lights are out, there's about 3-4 hours of solid quiet time before they start prepping for breakfast/coffee service. Not enough. When I moved to SF, the 10-12 hour flights to Europe turned out to be the perfect length.
* Time of flight helps. The best possible flight for sleeping for me are the 7pm departures out of SFO that land in Europe 10-12 hours later. That's basically normal sleep time for me, so my body is actually in sync with the flight.
* Supplements help. The magic combo for me is 3mg of melatonin + 1 glass of red wine :)