I remember a couple times where I did Whitney again a week or two after doing it the first time and the second time I didn't really feel the altitude. But generally what works for me and makes a HUGE difference is spending an extra day at altitude before the hike. I used to just drive from sea level to the campground the afternoon before the hike, but now I leave a day earlier and stop at Mammoth Mountain Inn along the way. Then stay at the campground the day before the hike (Horseshoe Meadows is another great option but I can't sleep as well there as I can in a hotel bed).
Also the better conditioned you are, the less your heart has to work and thus the less you'll feel the altitude.
Bingo. Time, time, time.
This is far more important than wimpy benefit of medical Diamox or unproven magic potions like gingko or beet juice
