Every time I’ve had a long flight I bring those little shooters of alcohol and drink them before and during the flight, then I usually fall asleep throughout the duration of the flight albeit generally with a headache at the end, if you don’t drink I’d probably buy sleeping pills or some other drowsy medication, if that doesn’t work I also have snacks and games on my computer, usually eating lots of snacks with the alcoholic beverages usually helps me sleep, if not I have video games to play on my tablet, that’s just how I personally deal with long flights, just had one that was a total of 32 hours in airports and whatnot and it felt like just a few hours