I'm a lowlander. Been living in chowder-like sea level atmosphere all my life. Rarely get above 6000 feet and the highest I've ever been in my adult life while standing on solid ground is a shade over 9000 feet.
I definitely begin to notice the thinning atmosphere at around 6000 feet and would say that it begins to affect my performance by 7-8000 feet.
So what's my best bet for dealing with the fact I'll be skiing above 8000 feet and as high as 11000 for a few days in February? I won't have more than 24 hours to acclimate to the higher elevation.
I figure the best I can do is go jogging and biking as much as possible and make sure my cardiovascular system is in as good of shape as it can be.
Is there anything else to do that will help? Maybe I could put my head in a plastic bag for a few seconds longer each day to simulate a thinner atmosphere and help me get used to it? :p