I find that it helps if I keep as close to my usual routine as possible - brushing teeth, removing contacts + makeup etc just before I sleep. Even if I'm wearing no makeup, the simple act of washing my face with warm water helps put me in a better mood to sleep. I read somewhere that if you keep to a pre-sleep routine daily, it's easier to fall asleep since your body and mind are accustomed to that ritual.
I try not to take any pills, because I want to be able to wake up easily and be alert in case anything happens.
I've managed to sleep at least 80% of flights that way.