I've actually slept nicely on an airplane; even resting the eyes are good for the muscles and the overall health of the traveler.
I typically listen to music I recognize while I attempt to sleep, regardless of where on the plane I am (even next to the engine). I prefer a window-seat, and I try to secure one as much as possible! Just listen to the music, close your eyes, and focus on the tones until it becomes white noise and you pass out.
1.) I'm usually dead tired anyway from all the hustle, especially since my flights tend to depart early in the morning.
2.) There's nothing else to do.
3.) Airplanes create a pretty sleep inducing atmosphere on calm flights. The air pressure, din of the engines, distant sounds of passengers and flight crew create a dream like quality.
Unless there's something disruptive going on like a screaming brat or a fatty sitting next to ya, the only obstacle is the atrociously uncomfortable seats. It helps to have a window seat so you can lean your head against the fuselage. I usually don't like putting my seat back because I don't want to be "that guy" and it's an invitation for the flight attendants to come over and complain at you. Otherwise I can never sleep if my head angles down.
On one flight the jerkass captain wouldn't stop using the in flight com to announce pointless blather which kept waking me up every 7-10 minutes.