It's important to get a couple lower hikes in the weeks before, like do a day-summit of jacinto one weekend, maybe gorgonio the next. Pacing yourself on the Whitney trip is key, too, and I find sleeping at the portal or even Horseshoe Meadows the night before and then heading over helps me.
I've hiked with folks on Diamox, it seems to make them tingly and food tastes like garbage. I would advise against that if you prepare yourself naturally otherwise.
This probably goes without saying, but you need to be used to the pack weight you'll be carrying, and do some general cardio workouts prior to as well.
