The decreased atmospheric pressure at high altitudes of 3000 feet & above affect the way hot air expands and how steam is created in baked goods. This causes your baked goods to rise way too much to the point that they collapse. Therefore...
1) You will need to reduce the amount of your chemical leavener (ex. baking soda, baking powder) by 1/3 for altitudes at 3500 feet, by 2/3 at altitudes over 5000 feet.
2) Your eggs should be under-whipped to avoid incorporating too much air, which would also create too much of a rise.
3) For your yeast-leavened products, you should bake them before they are fully proofed -OR- reduce the yeast by 20% when baking at altitudes over 5000 feet.
4) You need to increase your oven temperature by 25 degrees Fahrenheit (4 degrees Celsius) at altitudes over 3500 feet so that the structure of your baked good sets rapidly.
Also, the boiling point decreases at higher altitudes which causes more moisture to evaporate from your baked goods while they're in the oven. This will cause dryness and excessive sugar proportion in your products, which you will see as white spots on products like cake.
Therefore, to help adjust for this you will need to reduce every 8 ounces of sugar by 1/2 ounce at 3000 feet and by 1 ½ ounces at 7000 feet.
Water under normal atmospheric pressure at sea level boils at 212 degrees F. (100 degrees C.). The boiling point of water decreases as the altitude increases because of the drop in atmospheric pressure. So for every 1000 feet above sea level, the boiling point of water drops 2 degrees F. (ex: in Denver water boils at 203 degrees F.) Therefore, because the boiling temp is lower, it will take longer for foods to cook in high altitudes than at sea level.
Also keep in mind that if you are adding alcohol to the water, this will lower the boiling point even more because alcohol boils at around 175 degrees F. (@ sea level).
However, the addition of salt or sugar to your water will increase the boiling point; so foods cooked in salted water will cook faster because the boiling point is 1-2 degrees higher than normal.
3Unfortunately, adjusting typical recipes for high altitudes is somewhat risky and sometimes different types of baked goods will need different adjustments techniques.
You can also contact your state's Department of Agriculture for assistance.