Where do you live?  I have been taking a class called "Core Fusion" for almost a year now, and it has been the most effective thing I've done in terms of toning my body up (all over, not just butt).  It is offered at Exhale Spa, which has locations in Boston, New York, Chicago, Venice (CA) and Santa Monica - and I believe they'll be opening up a Dallas location in 2006.  \n\nIf you are not near an Exhale, yoga has a lot of postures that will help you tone your butt muscles (maybe check out a local gym or studio?).  So does Pilates (which I'm not really a fan of - I find it boring).  \n\nThere are also the old "standbys" of squats and lunges (you can get added benefit by integrating props or machines - i.e. do the lunges or squats on the Smith Machine - the one that has the bar with adjustable weights that you square on the back of your shoulders while doing the exercises, or stand with your back to the wall, place a physioball behind you and roll it up and down the wall with your back as you squat).  One important thing to remember about squats and lunges is not to let your knees go forward of your ankles - that puts a lot of strain on your knees and sets you up for unpleasant injuries.  \n\nIf you go to the gym, some of the weight machines they should have can be effective butt toners - like the leg/hamstring curl machines.
How do I tone my butt? What's the best exercise for butts?