I live at 6400 feet in a suburb of Denver, CO. Cakes that used to get rave reviews at sea level are turning out dry, not moist and dense, with air bubbles, and cook unevenly. If I check it with a toothpick, the center falls sometimes. Help!
Hi and Welcome to CC, McPenner. 
Decoding CC acronyms:
http://forum.cakecentral.com/cake-decorating-ftopict-2926-.html
High altitude recipe adjustments:
http://tinyurl.com/3d6p5s
Recipes for baking at (various levels) high altitude:
http://ces.uwyo.edu/PUBS/B427.pdf
http://www.cahe.nmsu.edu/pubs/_circulars/circ293.pdf
HTH
Silly me, I thought that was how a boxed cake mix was supposed to turn out! LOL! I will check out the links! I'm an hour north of Denver, and I never even bother with high altitude directions, perhaps it's time to save myself some big hassles!
I've recently moved ~1 hour north of denver (Windsor, CO) and moved my dessert making to focus on cakes & cake decorating & I thought I was just really bad with cakes 
but I hadn't changed a thing from recipes I had when living in Portland <LOL>
I'll try some of the recipies from the Wyoming link above & let you know how it goes! 
Hehehehe.........6400 is not high elevation
I have been at 8100 ft elevation for the past 5 months. I have baked about 6 cakes in that time. The only change I had to make was add about 1/2 cup more liquid (mostly used Betty Crocker boxed mixes - but did one choco hi altitude scratch recipe) and bake much longer - like 45 minutes for a 10"x2" round.
I'll check out those links too, but in Loveland, CO, the only adjustment I've made is to bake at 325 degrees instead of 350.
Hi there. I'm in a Denver suburb too. All of the suggestions you are getting are really good. Are you baking with mixes or from scratch? Mixes usually come with high altitude instructions. I'm strictly a scratch baker myself and I don't have too many problems with cakes up here.
Definitely allow your cakes to cook a little longer. If you try to poke 'em with a toothpick before the center is set, they will fall. Also, and this has nothing to do with altitude, check your oven temperature. I bought an oven thermometer about a year ago and discovered that my oven runs about 25 degrees cooler than it says. Now, I always set it 25 degrees hotter than a recipe calls for.
Also, don't over mix your batter. That's what will cause cakes to become dry or tough. Also, it really helps to measure your flour by weight rather than volume. It's so dry here that you can end up with a lot more flour in your recipe than you realize.
Good luck!
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