If you are growing from clones you will allways have problems with it since powdery mildew is systemic, it's kind of like plant herpes its there all the time but only shows up when conditions are rite for it. There are precautions you can take like keeping temp and humidity in the proper ranges, running a sulfur vaporizer during veg and h2o2 washes but even then it can show up. If you are growing from seed and keep getting it you need to take your grow room apart and sterilize everything from floor to the cealing lights pots fans everything that will be in your room. I have heard there are systemic soil drenches you can use but I have no experience with them and don't really know much about them.