I skate hills so my shits flatspot right away usually. I bought a set of 57 mm spitfires a month ago, Skated some hills the first day, went to a park on day 2, hoping for that new wheels on skatepark ground glide, and it felt like I was skating on bricks. Luckily, I can usually work those out pretty quick too. One bad powerslide= flatspots, a couple good powerslides= flatspots gone. I usually get new wheels when my 57's look like fat little chode 48's. I should rotate my wheels, but I don't, so my back right wheel is a few mm smaller than the rest. Its usually a month or 2. I would every week if I could afford it.
Sounds like you need soft wheels. They'll give you grip at the parks, and the rough ground will be more forgiving. Sliding on your wheels will be a little more difficult though, so be careful.
I always get spitfires, because they sell 'em a bit cheaper at the deluxe store. I hear the powell wheel formulas are pretty good too though.