Basic principles I follow on flights that are over 8 hours in length:

1. Avoid alcohol and caffeine
2. Try to increase salt consumption slightly to help reduce dehydration
3. Increase consumption of water to 2 to 3 times what you'd normally drink. This won't cause excessive trips to the bathroom because you really do dehydrate quite a bit at altitude
4. Avoid eating protein and foods with high fiber levels; try to eat a light meal of carbs instead to ease digestive burden.
5. Put on a "Breathe Right" strip. They look ridiculous but do help
6. Change into very comfy clothes. I usually change into a t-shirt and umbros before boarding. I also take off my shoes and use the slippers the airline provides. Use a blanket to keep warm if cold.
7. Don't knock yourself out TOO early in the flight. If you wake up with 2+ hours to go you've pretty much nullified all of your efforts.
8. I like using Benadryl as a knock-out. I find the sleep is more restful than the Rx sleep aids like Ativan and Sonata (which are really watered down anesthetics). The draw back of Benadryl is that you have to calibrate your dosage level and time carefully to prevent early wake-up.
9. If you have a layover, take a shower in the lounge. This is especially important if Narita or Hong Kong or Amsterdam are the transfers (they have nice showers available for first class passengers).

The basic idea is to fool your body into thinking you've had a full night of sleep. Being as comfortable as possible helps ensure this.

I generally look ludicrous wearing my thick glasses, a breathe right strip, umbros, and several blankets as scarves and a make-shift "dress". However, it always seems to work for me.