I really struggle to sleep on long-haul flights, so always ensure I've got plenty of things with me to keep me entertained. Make sure you take a selection of books, magazines, sudoku puzzles and crossword books. Also, if you can upgrade the entertainment on offer to include the likes of "who wants to be a millionaire" games, do so - anything to stave off the dreaded boredom!
Do NOT to take a sedative - the crew may think you unwell & should anything serious happen you would be in no fit state to evacuate the aircraft. Just enjoy the experience & accept it for what it is - e.g. if your first choice of meal is no longer available or you are disappointed with the amount of leg room then just remember that you are cruising at 30,000ft & so no matter how stressed you get that meal or extra leg room still cannot be conjured up. Flying is an amazing experience - it's made the World a very small place, there are good movies & food to enjoy, & the crew will always help you if they possibly can. I think if you just remember these things & appreciate all that flying CAN offer you then the experience can be very enjoyable.
It really depends on how how stressed you get. Most long haul flights over films and entertainment on board so you should definately take advantage of those. Also it is best if you try and get a seat nearest the end of the row (next to the space between groups of seats) so that you can take a walk every hour. Try an MP3 player filled with relaxing tunes or your favourite hits, puzzlebooks are also good but a good idea would be a banana and a small flask of warm milk to help you get a more natural sleep.