I think that the only way to adapt is to maintain a good overall level of aerobic fitness and to ease into any altitude gain as gradually as possible. It also helps to stay well hydrated for the first few days of a trip. Drinking alcoholic beverages should probably be curtailed over this interval as well. Resting for a day or two before starting the hike is also a good idea.

For me, aerobic fitness comes from cycling or fast walking at least an hour, six days a week and day-hiking once or twice a week in the 9,000'+ mountains around where I live. I almost always enter the Sierra from the east side so easing into altitude is not always practical so I just have to suck it up a bit.

I lived in Flagstaff, AZ (7000') for six years and though I don't think there is any residual physical benefit, I am certainly familiar with the effects of altitude. For this reason, I don't pay as much attention to them as I suspect I would had I never lived at altitude.