I've done many long-haul, international flights over the years. For me some of my best personal tips include:
1) As others have so rightly suggested, always be prepared with your own entertainment. I always bring a new book, my kindle, my iPod/iPad with movies, tv shows, and games on it. I've found that many of the planes of US carriers (at least, American and United) have outdated entertainment options. By that I mean that many of the planes do not offer personal, touchscreen entertainment options in the seat head in front of you. Most European and Asian carriers are much better in this department, I have found.
2) I always take sleep aids, just in case I feel like I need them. This is obviously a personal choice. I normally just bring a melatonin or two Advil PM's just in case.
3) Going off the point above, in addition to the sleep aid, I always have a little pack of Tylenol or Motrin, an antihistamine, and imodium, etc. just in case I start to feel a little under the weather or have a headache or something while I'm flying. A good nasal spray too is something I would tend to carry. I always get very dry when traveling (especially for extended periods), so this helps a lot for me.
3) For me, I always bring a sleep mask as well. To some this may seem (or look!) slightly ridiculous, but for me it definitely helps me to nod off more because it allows me to cancel out a lot of the movements and lights, etc. of being on the plane.
4) Always bring a spare pair of clothes in your carry-on just in case your luggage gets lost. In addition, I always have a little toilet article bag where I bring travel size deodorant, mouthwash, toothbrush and paste, etc. That way I can freshen up if I feel like it. Especially if it's been hours and hours and we're about to reach our destination.
5) Again, if possible, try to fly business class. The extra room, service, and lie flat (or near lie flat seats) will make a big difference to your overall experience. Try to use miles or if able, then pay for it outright. Obviously, this isn't available all the time for most of us (like me), but if you can swing it, then I say go for it! Treat yourself a little! LOL
6) This one might sound a little strange, but have the appropriate mindset going into your journey. If you know you're going to be flying to Asia or wherever long-haul, then I find it helps for me just to prepare a little mentally. No one wants to be sitting on a plane for 15 hours, but I find it helps if you just accept that fact that you're going to be on the plane a long time and just relax. It will make the flight go by quick and you'll enjoy yourself more if you're not constantly looking at the clock and counting down how much more time you have left.
7) If you're traveling solo, then I always try to at least be cordial to the people sitting next to me. A simple hello can go a long way. Obviously, some people don't want to talk and that's fine. However, I've had some really lovely and interesting conservations with passengers seated next to me that proved to not only be enjoyable, but also helped to pass the time away as well.
8) Get up and walk around. You NEED to stretch your legs and get the blood flowing, so get up several times throughout your journey and just have a stroll around the cabin. I'll often times go to the flight attendants' area and ask for a glass of water and stand and just stretch for a few minutes. Sometimes I'll strike up a conversation with the flight attendants, which again helps to pass the time, but also it's a great way to tap into their knowledge about the destination that you're going to. Chances are they've been there before and can offer some insightful tips on things to do, places to see, restaurants to eat at, etc.
9) Also, I always bring one of those personal electronic chargers things. I don't know what the actual name of them are. But the devices where you can charge them overnight in the wall or plugged into a laptop or something and then can plug your phone or iPod, iPad, etc. into them and charge them back up again. This is especially good if there are no outlets in your seat, or if you're always on your phone (like me) for one reason or the other! Also invaluable to have once you arrive at your destination and are out and about sightseeing all day long!
Sorry there is no really 'logical' order to these points. I just kept numbering as more things popped into my mind, haha.
Hope these are somewhat helpful- Happy flying everyone!!