Sleeping on long flights - What's the secret?

Hi,

I'm going on some long flights soon, and would
really like to do some sleeping during the flights.

But somehow, I can't seem to get comfortable enough
to get to sleep. I'm 6'3" 280 lbs, and the tiny
seats are -very- uncomfortable after a few hours.
(can't afford 1st class)

Can anyone suggest a method or even proper drugs
I can use to get to sleep so the flights don't seem
so long?

Thanks for answering here...