You can do all the normal things we recommend to mitigate powdery mildew: plant in full sun, make sure the plant has good air circulation by keeping it properly spaced from other plants and structures, and avoid overhead watering. Your peony rings also help a lot by keeping foliage off the ground and allowing air flow to the whole plant. However, even if you do all the best practices, the simple fact of the matter is that peonies just get powdery mildew. To our knowledge, there are no herbaceous peonies out there that are resistant, and I can’t recall any not ending up with at least some powdery by the end of the season. So, don’t blame yourself, and take comfort in the fact that while it may be a bit unsightly, powdery mildew does not harm or set back the plant at all. One last bit of advice: come fall, when the plant is dormant, cut off and discard all of the brown foliage.