There have been a few books I've read that talk about how to choose volunteers for your act. I really liked the suggestions for it in "
How to Pick Pockets for Fun and Profit". The following is a quick run down from the book. Share your tips here.
Avoid the clown or the loudmouth who will want to steal the show or who will try to make your task difficult in order to show off and look clever.
Avoid those who appear timid or shy. These people may be nervous or uncomfortable on stage.
Choose people who are friendly, outgoing and not afraid of getting up in front of an audience.
Never choose a volunteer who is intoxicated or who appears to be a heckler or show off.
Before the show starts look over the audience for likely volunteers.
Look for people who are part of a group. Choosing a person from a group will enhance the audience's reaction to your stunts. The fact that someone the group knows is on stage makes it more enjoyable for the group and they will respond more freely. Their reaction will spark increased laughter among the rest of the crowd thus increasing everyone's enjoyment.
The response is even greater if the subject you choose is a leader or popular member of the group. His friends will love to kid him about it later.