I am at 4800' and I usually let rise longer and they turn out fine. I always use bread flour, that may help. If all else fails contact your local county extension office and usually they have suggestions for baking at high altitude.
I found this on Epicurious
Yeast Breads
At high altitude, bread tends to rise much more rapidly than at sea level and changes in ingredients or technique are needed to slow down this action. Some bakers reduce their yeast slightly or use ice water instead of warm water, while others punch down their dough more often, then add extra rises or one overnight rise in the refrigerator. Beware of dough that has risen too much or "over-proofed" before baking; it may warp, droop, or collapse in the oven. To prevent over-proofing at high altitude, only allow dough to rise about a thirdnot double in bulkbefore baking. Never omit salt: At high altitudes, salt is essential not only for flavor, but also to slow down the growth of yeast and the expansion of gases. To achieve good rise and a crisp crust at high altitude, begin baking bread with a pan of boiling water on the bottom of the oven, then remove the water for the final 15 minutes of baking.
Muffins & Quick Breads
This category also includes scones, biscuits, and cornbread. For the correct rise at high altitude, baking powder or baking soda must be reduced slightly. Also, you get a better rise and quicker set with an acidic batter, so you can reduce some of the baking soda, which neutralizes aciditydon't omit all of it as some is needed for leavening. To strengthen batter and prevent collapse, sugar is reduced and flour increased. Extra liquid can be added (it is especially good to substitute buttermilk or yogurt for water or milk) to compensate for dry air and drier flour at altitude. Quick breads baked in loaf pans may crust over and start to brown on top before the batter underneath begins to set. To prevent this, sometimes it is helpful to cover the pan loosely with foil after half the baking time or to substitute a tube pan for a loaf pan. To get a better rise between 4,000 and 6,000 feet, increase oven temperature by 15°F to 25°F. At higher elevations, keep original heat and increase baking time slightly.
Read More
http://www.epicurious.com/articlesguides/howtocook/primers/altitudebaking_basics#ixzz0wJJjITxN
