If you wear neoprene (wetsuit material) give it a handwash in warm water with something like McNett Wet & Dry Suit Shampoo. Line dry thoroughly, turning it inside out so there are no damp folds.
You might try wearing synthetic briefs under the wetsuit. (Cotton absorbs water, gets soggy, and can make the problem worse.)
Don't grease up with petroleum jelly or any gooey stuff, as it gums up the neoprene and is really hard to get out. Medicated talc (not cornstarch) powder is good. When you take off the neoprene at the end of a day, wash your butt, air dry, and apply some sort of powder. If you wear briefs, change them. If clean water is short, take a couple packs of those medicated moist wipes. If you're on a trip, rinse the groin area of your shorts or wetsuit in clean water (i.e. not straight out of a silty river) and air dry overnight.
On hot trips, for sleeping either wear nothing or only synthetic wicking-type briefs, and powder up before hitting the sack.
If you do all these things and still have rafter's butt, you're cursed by the river gods.