Several of us just returned from Colorado and did climbs up to 12,000+ feet on mulitple occassions over a week. It's important to note than in general the climbs are not that steep-usually less than 8%. But they can be longer than what you might have around where you live. I've never really had the higher altitude affect me that much while riding except for some general light headedness bending over to get my water bottle. For me the key is to just get into as good a riding shape as I can get into. Do as many hills as you can just to make sure you're training the muscles you'll be using more off. Make sure you have easy enough gearing that you can spin for the long climbs. If you use too hard of gearing your knees will start to suffer. It not only helps your legs to do a lot of climbing it helps your other body parts that will be in more of climbing position-like your back and arms. Be prepared for a little more wind than you might be accustomed to as well. There is less to break the wind and it just seems to hinder you more out there than it helps. Ride within yourself and you'll do fine.

I also tried the stairs when I first starting riding and to be honest I don't know that they helped that much. There is no substitute for simply getting time in the saddle. Riding into headwinds and using a little harder gear than normal seem to be better surrogates for hill work if you don't have hills to train on.