Sarah- if you have the heart to do so, you can sort of prune them down a bit maybe in early July I'm thinking for your zone, and the plus is you can propagate the cuttings for more plants and extend your season into Fall. Another method I try to stick to is to remove any seed pods that start forming by hand (tedious, I know) so the plants don't bolt too fast in the heat. You might want to try this method on just a couple of plants to see what the results will be for your own garden.
-Nan

Thanks so much, Nan...I'll try your suggestions!

Pinch, pinch, pinch!!! Keep pinched portions in a glass of water til roots form and then plant them, you do two things, keep "mother" plant nice and full not leggy and you make tons of new plants for your garden as well. You can use Pinch and Poke method also. Pinching Impatiens is almost a necessity if you want them beautiful in August/Sep.
Pauline