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.