The way many people solve this, or at least attempt to, is by using a neck support, and then alcohol or drugs. Ideally if you take any form of sleep inducing pill, you should take it after you take off in case the flight returns to the gate and you need to deboard. If you do choose to take this route, I'd recommend trying your chosen strategy before your first attempt on a long haul flight, in case you have a negative reaction in some way.
It's definitely not recommended getting drunk before a flight as you may be refused boarding, however 2 or 3 drinks may sufficiently relax you in order to assist with sleeping. Only you will know how that works for you.
It's also worth considering that if you fly often, there may be ways to get reasonably priced business class flights or ways to upgrade to business class in order to get a flat bed. I'm currently on a trip in business class that cost less than economy would have been without a Saturday night stay. I was willing to give up a day of my weekend in order to have a bed.