Here are the first ten that come to mind:
1) A few days beforehand, start taking aspirin. Also, start boosting your immune system by taking extra vitamin c ... I use Emergen-C packets.
2) 24 hours before you take off, take a decongestant to clear your nasal passages and sinuses. Less pressure and pain during the flight.
3) Obviously, drink lots of water to stay hydrated
4) Avoid alcohol and caffeine, while on the flight
5) Don't cross your legs when you sit, if possible
6) Stretch every thirty minutes, then, get up and walk, stretch, exercise, do airplane yoga (for real) between meals.
7) Bring earplugs
8) Bring Benadryl, just in case you wanna knock yourself out
9) Bring a kit with some fun travel toiletries in it, and use it at least a few times during the flight; good moisturizer (yep, keep slathering that all over yourself), eye drops, toothpaste and floss, pre-moistened face wipes, lip balm, gum. Basically, you want to stay hydrated and clean
10) The standard neck pillow, magazines, books, ipods, dvd players etc. will take you really far, but good snacks that don't make you feel bloated, will go a long, long way too. Hit Trader Joe's or Whole Foods before you go and don't go get a bunch of fiber either ; ) Simple, high protein snack are the best. You'll save tons of cash and save yourself from airport food.