In our area, where we are exposed to a lot of heat, humidity and ocean breezes, mildew can be a challenge. Mildew is an airborne spore that lands and stays on everything that isn't washed frequently. Contrary to some peoples belief paint does not cause mildew. Mildew grows on concrete, roofs, fences, cars and probably even us if we didn't bathe. When starting an exterior painting or staining project you must start with a clean surface. Use 3 parts water and 1 part bleach to remove existing mildew, and mildew spores are there even if you don't see them. Only clean what can be painted in a week or two. Use a high quality exterior paint that has a good mildewcide in it, this will help prevent the mildew from taking root in your paint. There is a difference in mildewcides and we have had the best luck with Benjamin Moore Paints. If you have had chronic mildew problems I would add additional mildewcide to the paint just to help out. If however the mildew is under the existing paint there is very little that can be done to stop it, short of stripping the paint off and starting over.