Make sure its powdery mildew and not downy mildew... they look similar, one is white and one is more yellow and the downy spread MUCH faster.
Catch it and tree it often and early. Try Copper Sulfate if you are not organic, or neem oil if you are organic (just be careful about treatment with neem as it is also a natural pesticide and can harm bees when its freshly applied). You will probably not be completely rid of it, but you will be able to keep it under control with regular appliations of said products.
If its barely there when viewing, try a 1/4 milk to 3/4 water solution to start you off. Milk is good against mold/mildews.