altitude acclimatization

Any specific suggestions for preparing for a marathon at a higher altitude than one is used to? I live, train, and run about 35 miles a week at sea level, and will be participating in the 2007 Colorado Colfax Marathon. The air in Denver is, of course, much thinner than here. I'll arrive in Denver three or four days before the race. Generally speaking, is that enough time to acclimate? Anything else I can do in advance to prep? Thanks!