I've never been able to sleep on a plane, I've flown to the US about 4 times now and everytime I just can't doze off.
It's mainly because I have sensitive hearing I think, the background noise of the planes air con (An old British airways 777 way beyond it's time!) that keeps me awake.
I bought a pair of noise cancelling headphones, but they only stay on when something is being played through them, so I might try finding a long background noise track to see if it helps me fall asleep.