During the day before the flight get exercise. Eat dinner in the airport before a night flight (avoiding heavy steak, Espresso Martinis and pudding) . I go to the loo and brush my teeth before departure (just before boarding if in Y, on boarding if premium cabin). In other words have a sleep routine that's as close as possible to your normal one and train your body to it. I don't eat the dinner (or equivalent) on the aircraft.
If you can, when (assuming it's night) the lights are dimmed for take-off be ready to go to sleep before they are turned back up for the service at a few thousand feet. So your body gets the message of "it's gone dark must be time for sleep." I know some people in biz / first put PJ's or similar on before departure but for me I'm always in "run away" mode until off the ground. So I tend to wear comfortable walking trousers I can sleep in to save changing (and have a cargo pocket for my passport etc). Soon the gear comes up I'm off with my shoes, on with the eye mask and ear plugs and trying to sleep (flat if possible) before those overhead lights come on.
I take an extra neck pillow, even in business. Sometimes carry a cotton sleeping bag liner. Depends on the route, some seats are pretty gross.
Oh and I write a note and stick it to the side of my seat or (on my eye mask in economy) ... might be "please do not wake for food" or "wake for breakfast" or something, depending on duration. Finnair in biz actively collect cards asking if you want woken up for breakfast.
Getting to sleep is one thing. Staying asleep can be just as challenging. I have to really fight the urge to peak out the windows. There are some amazing sights and when you're staring down at the world below in daylight your body is not producing sleep hormones. Dehydration can be an issue for staying asleep, I carry my own water bottle with a sports lid...I can locate and drink without having to take my eye-mask off. Keep the body thinking it's dark, even if it's not.
Personal preference but I also like older aircraft like the A330 as there is more background noise to drown out the snoring of the other passengers!
Not my idea but a guy I was sat next to one day in Premium Economy on BA was carrying a rolled up two inch thick foam mat, cut to the shape of a BA seat. He worked as a courier or something and did huge numbers of Econ flights. Now that's planning!
Not my idea but a neat way to improve the PE seat.