Three things that are really helpful are those contoured neck pillows. I have an inflatable one as it takes up on space and I can underinflate it so my neck fits in it as I want. Also, foam in ear plugs. You squish them down and plunk them right into your ear canal. They expand and block out pretty much all sound. The last thing I suggest is an eye cover. It will keep things nice and dark for you.

Another thing you can do is before your flight start thinking about how nice it will be to sleep on the plane. Think of how you're going to go on there and have a nice deep nap. When you do that, your body will start to realize that its time to go to sleep.

Combine that with skipping the alcohol and caffeine and a sleeping pill and you're off.