I have the same problem-I get rashes, irritation, and yeast infections if I don't change and shower IMMEDIATELY after working out.
I have found not wearing any undies & lose shorts helps, as does (ok, I know this will sound funny, but it works) a product called Butt Paste (you can get it at Target by the baby products by the Pharmaceuticals).
It minimizes irritation, prevents chafing, and soothes the skin. I would avoid using Cornstarch and (especially) Talc, as these mixed with sweat are going to make a paste!