I never used to be able to sleep on airplanes, but I seem to have my formula down now: earplugs, eyemask, and get the feet off the floor!

That last part is really important because it helps isolate me from the vibration of the plane. I also like to put a bag or something under my knees to keep the edge of the seat from hitting a pressure point back there. My backpack only does a fair job at this. If I could invent something, it would be a unit that goes under your legs/feet to accomplish those two things. It would need to fit under the seat in front of you and ideally it would be dual usage as luggage or something.

plane sleepy-by time
 

I swear by Silencio brand earplugs, available on the Amazon website. I fly across the Pacific and value my quiet sleep time. They are silicon filled, so need to be warmed up so that they will shape and conform to your inner ear. Once in, dampened with a little water to effect the seal, you can have a screaming baby behind you and not care. Between those and my 5 mg of Ambien, I get a solid 7 or 8 hours of uninterrupted sleep. Oh, I also bring along my own small crushable down pillow in a carryon bag, and use it so sleep with against the bulkhead, as well as for the comfort of home in the overseas hotel room.