Powdery mildew happens to be one of my most hated of plant diseases. It's a fungus that rears its ugly head come spring and it's spread by spores that hang out in your soil or travel to your garden via wind and birds. Basically, it sucks.
The first line of defense is to grow disease resistant plants. I've seen powdery mildew affecting everything from forget-me-nots and roses, to boxwood and bee balm. Do some research on varieties that are less likely to get infected.
Nip it in the bud. Remove affected leaves and buds once you detect the fungus. Spray with a fungicide of your choice. I've had good results with neem and a homemade garlic spray. Don't let diseased leaves sit around in your yard. Rake them up and get rid of them. (Don't put them in your compost, you'll just spread it back into the garden).
Water in the morning and water the base of the plant allowing the leaves to stay dry. Powdery mildew likes humidity and heat and if you overhead water in the summer you'll increase the chance of spreading the disease.
If you just can't get rid of it, consider removing the plant and replacing it with that pretty Japanese iris you've been admiring in your neighbor's yard...