First, I try to grow varieties that have some mildew resistance. I'll buy seed that lists resistance. And when I save my own seed, I remove those plants that get mildew the worse, thus ending up with seed from the most resistant plants.
Next, I try to catch mildew when it first starts. Sometimes the plants start showing off colored spots or splotches on their leaves before they turn whitish. If I can start spraying at this early point, I can battle the mildew better. On some plants, like the squash/gourd/pumpkin family and the tomatoes, I'll also remove infected leaves, then fertilize the plants. The plants then produce news ones.
Most likely because of my location and climate, I haven't found that spacing the plants further apart or doing something else to open the plants up to more air and sunshine to help with the mildew. I've tried, but haven't seen any difference.
I've tried milk sprays but the jury is still out. I haven't yet done any controlled trials, so possibly milk spray might help some if the mildew is caught in the early stages. I've tried IMO foliar sprays. Just as with the milk sprays, the jury is still out. But I get the feeling that it works better than milk.
I have been using baking soda spray. So far it seems to help the most, but it's not the perfect answer either. The spray I'm currently using is--
1 tablespoon baking soda
2 tablespoons vegetable oil
1 squirt (about a teaspoon) of liquid soap
1 gallon water
Mix together the oil and soap, then add that to the gallon of water. Shake. Add the baking soda and shake well before using. Spray the entire plant, top & bottom of the leaves. Because this pray contains oil, do not spray in bright sunlight. I spray it in the evening just before dark or very first thing in the morning right at dawn. I'll use it once a week, but if it rains, then after each rain.
Mildew has to be treated as soon as it starts showing up. Once a plant gets whitish all over, it's too late.
I plan to try experimenting more with mildew control. Mouthwash, vinegar, and sulfur have been suggested. I haven't tried any of these yet. I'm going to combine foliar feeding along with various spray combinations. And I plan to try sour milk mixed with compost tea. who knows if any of this stuff will make any difference. This year I'll try to keep some better records.
Some of the records I have actually kept was trying to figure out if powdery mildew occurred during a certain time of the year. If it did, then I'd be able to take preventative action. But alas, that's not the case. I suppose an outbreak is more dependent upon weather conditions rather than the date. So I need to do a lot more close observation to figure this thing out.