I've used the balloon and t-shirt method on several occasions and it's not bad. However, I believe that balloons in the torso could also create a false sense of when to stop shooting. If you put the round right where it needs to go, the balloon pops and you stop shooting. If you put a handgun round into an attacker's heart, that is no guarantee that you have stopped him. He may have many seconds left in which to "stop" you (Miami FBI shootout). His head also may not be a target of opportunity at the moment you need it.
I really like the idea mentioned about using a partner to drop the target when enough good hits have been placed. I think that is an excellent idea and could be done with some imagination and minimal effort. This would allow for the constant changing up of the number of rounds and shot placement so that no one drill is overly imprinted (to the exclusion of others) into the mid-brain.