cycling is the best thing evar for yer butt,, there are also pilates movements that target deep butt muscles (note the technical terminology) which you can modify to work even harder using a flexband: the clam! the elevated clam! the side clam using a flexband.
When I was in physio. The guy said for butt and thighs lay on your stomache, put some weights in a bag (Or get the kind that wrap around your ankles) and keeping your leg straight lift it straight up. Then do the same laying on both sides and your back.