Plyometrics are great for your butt!
Squat jumps like those described by Miss Harley are great. If you really want to 'feel' it working, don't hold the squat. When you jump down, jump straight back up. Remember to keep your form good when you land (chest up, butt poking out), and thrust forward with your glutes for extra power when you jump. Also make sure you jump from your heels NOT your toes- other wise you aren't working your butt as much as you should.
Scissors lunges are also great.
There is another good one that is like sitting on a chair, but as soon as your butt skims the seat, you jump right back up.