If you're part of the no-whale-tail group, but still want to wear those ultra-lowrise jeans and pants, you can try these "backless thongs" (because really now, it's not a good idea to go girl-commando in jeans). The thing, er, thong, is "specially designed with adjustable leg bands that lift and flatter your bottom. Simple and discreet, these leg bands also hold Backless Lingerie in place, removing the need for an uncomfortable and revealing g-string." They say it works well for those form-fitting even dresses as well.
http://www.styledash.com/2006/09/15/backless-underwear/