"What can I do to help prevent this?"

Nothing!

In my experience when I was selling that stuff---at Paulino's---I never saw it without mildew! When it was blooming we'd just strip the dead/dying/ really awful looking! leaves off of the stems and, believe it or not, the customers would be so transfixed by the flowers that they didn't even seem to notice that all that was there was a bare stem with a flower on the top! When the plants would be VERY young, they'd start out without the mildew, but by the time they were blooming, they always had it! There are some varieties that are "said to be" mildew resistant, but as near as I could tell, they were just as bad as the "other" varieties! Whenever somebody would take the time to ask me about the plant, I'd warn them about the problem, but most people were so mesmerized by the flowers that it was like selective attention, and they'd just ignore the warning!

My advice, if you want to keep it, would be to keep it REALLY well watered (they love water), and be sure it has as much air circulation as possible--in other words, keep it as "stress free" as possible, but I suspect that by the time it's blooming you'll still have mildew. You could try to treat it with a fungicide starting BEFORE the mildew even shows up, but, again, in my experience, none of the fungicides I ever tried (at work or at home) ever helped much. If you go that route, I recommend applying it at least a couple times a week! Once you can see mildew, it's too late!

The best you can do for your plant now is, as you said you were gonna do, strip off all the leaves and just let yourself be mesmerized by the flowers---and ignore the bare stems!

Sorry for the bad news,
Skybird

P.S. I love the flowers too, but because of what I saw when I was selling them, I'll never put one in! The more mildew I can keep out my yard, the better, as far as I'm concerned.

Plant a resistant variety. I'm using 'Blaustumpf' and another that the tag got thrown away & might remember if I see it, but the earwigs think they are a delicacy worthy of Batali, Julia Child or Giada.

Dan

Thanks I bought it for the bird and butterflies so I will just keep it and strip it! It hasn't bloomed yet.