I do a 12hr flight a few times a year and there is nothing worse than getting on the plane, having a few beers, some food watch a couple of films, read for a while and then realise your not even halfway yet! (it may be a 12hr flight but my door to door time is around 23hrs!)
I tend to struggle to sleep on flights, although it can be dependant on when I fly, from the UK going East it’s easier to sleep if it’s an evening flight but tends to mess you up for a few days as you arrive out there the following evening.
Coming back they are morning flights so you end up with 2 options, either hit the bars for an all nighter and hit the plane knackered and slightly pissed which means you sleep well, or hope there is a good selection of films and you have a few books to keep you going for about 11 of the 12 hours (as I really struggle to sleep in that instance).
The main thing for me though are noise cancelling headphones, they make a huge difference, a kindle with a big selection of books and a few films on the laptop just in case the choices on the plane are poor.
Always have an aisle seat, that way you can get up and walk around if needs be, window seats can sometimes feel a bit cramped for a long haul flight.