Hi Yorkie,
I have had this problem previously, under the tum, boobs and between the thighs. As everyone else has said, the thing is to keep it dry, and as cool as possible. I took to taking a small muslim cloth everywhere with me, in order that I could wipe down these areas whenever I went to the loo. However once you have the problem it can be really sore and difficult to get rid of. Having tried loads of things I found sudocreme best, but because I found this felt a bit yuky/sticky, I also used to coat that in talc. I found the best talc for the job, was one of those for femine hygenie, I think I got mine from boots. It has a finer conistancy than say baby talc and just feels better. I can't use perfumed stuff anyway, but I would suggest you steer clear, as this could cause more iritation. Finally, wear natural fibers where possible.
Hope this helps
W