My secret is that I usually don't get a lot of sleep the night before a flight (even if the flight is in the evening), so by the time I get on the plane, I'm usually pretty darn tired. If you're not averse to alcohol, you could also try downing one or two glasses of <something>, as that should help... but drinking to excess may end up causing you to be restless so I'd recommend only a couple of servings.

Of course, plenty of people take other sleep aids like Ambien... I'm not a subscriber to the prescription method, but that's another avenue.