Depends on how bad the dent is. I have a piece of wood that I covered with a piece of leather. I rest the outside dented portion of the shell on the leather and I tap the dent from the inside with a rubber mallet. If you're careful, you can make dents almost disappear completely. If you don't have confidence in yourself to do it right, take it to a horn/brass instrument repair guy. They take dents out of metal all the time.

John