I agree with most of the above information, but have a few tidbits to add:
1. The thing that got me from Montreal to Melbourne and back with little or no pain was a tiny pill available at most travel and health food stores called "No Jet-Lag." This is a homeopathic preparation and you have to take a pill every 2 hours, but I swear it works. It helps to prevent jet-lag and the bonus is that it makes you feel very sleepy and entirely relaxed. I'm a medical doctor and don't usually believe in or endorse this stuff, but I'm telling you, it's pure gold.
2. Sensory deprivation: bring a pair of earplugs, an eye mask, and an inflatable pillow for comfort (they always run out of the tiny, flat, white synthetic ones, anyway).
3. DVTs or deep vein thrombosis: this is basically what happens when blood clots form in the lower legs or more rarely, the pelvis. The clots themselves can cause swelling, pain and local redness, but the real danger is when they break off into the circulation ("embolize")and lodge in distant organs. The most common site is the lungs, producing a potentially fatal condition known as a pulmonary embolism. It would be virtually impossible to get a stroke from these blood clots, as they are in the venous system, not the arterial one, but I digress...
4. Air New Zealand is the best airline to fly to the Pacific Rim and beyond. The stewardesses are gorgeous, the seats recline all the way, the headrests are moveable and the dishes and cutlery are REAL.
5. Oh, yeah, and avoid EtOH and caffeine and drink lots of H2O.