If you are in a lie-flat business class seat you won’t have anything to worry about. But with J seats becoming harder to redeem, sooner or later you will find yourself in the economy section of a long haul flight.
Conde Nast has some ideas to help you out.
What worked best for me was getting a little help from my Dr in the form of sleeping pills and putting a boring movie or tv show on. Failing that, download a podcast on quantum physics or similar! It also helps if you are physically tired. We always go into town and look around on long layovers.