You'll need to know how many feet above sea level you are, because it makes a difference. I live at around 6300. This is pasted from a post I wrote to someone else today 
Hope it helps!
Baking at higher altitudes is certainly different. Through trial and error I have found what, for the most part, works for me. I bake both from scratch and mixes. I've found that some scratch recipes simply don't work, no matter what I do. The book Chocolate Snowball by Lettie Halloran Flatt has been really helpful to me. She lives and works at Deer Valley, Utah and is the executive pastry chef at the resort there.
At the most basic level, you'll need to:
decrease sugar by 2 tbs per cup
increase liquid by 2 tbs per cup
decrease leavening by 1/4 tsp per tsp
I've also recently heard that adding an egg may help.
When using a cake mix (straight or doctored) add 1/4 cup of flour and increase liquid as stated above even if the box says you don't need to.
It also helps to separate your eggs; beat the whites to stiff peaks; fold them into the batter at the end. I don't always do this, just sometimes.
I bake at a slightly lower temp (325) for a slightly shorter time. If a recipe says 35-40 minutes, I start checking at 25 minutes. I've recently read of baking at higher temp but since the lower has always worked for me I'm not going to bother to try it.
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