Sudocrem's the only thing that works for me. That and airing out. In terms of prevention, in this heat I switch from plastic backed products to cloth backed with a booster (I find the Abena M4's are the best cloth backed ones you can get). Plus, wearing gym shorts or something loose over the top. Sadly I think rashes are sort of an inevitability in this heat - powder's the only thing I've found which really reduces it.