over the winter, i was planning to go to some russian baths, but i didn't have a bathing suit that actually fit my butt. i ended up wearing an old pair of black dance shorts that i bought years ago to wear under costumes and whatnot: http://www.dancewearsolutions.com/dancewear/bottoms/shorts/TB113.aspx
they're not the cutest thing in the world, but they're short shorts that are designed to stay in place like no bathing suit ever will.
also, the butt's butt is no stranger to anyone who's ever had to wear a leotard--we used to call it "double butt" in ballet class (meaning the roughly 1/2 of each cheek left uncovered by one's leo). pretty much everyone had it.