Most of what you'll learn here is more accurately a "hypnotism act".
Many of the stage acts involve whispering in their ear as they are ushered up on stage "Play along, and we'll have some fun..." Those who don't play act well enough are usually returned to their seats.
DaveV...this simply isn't true...as a matter of fact, it couldn't be FURTHER from the Truth! You never have to tell audience members to play along...hypnosis is a natural phenomenon and it's quite easy to perform...I mean sure...it doesn't necessarily always work on everyone you bring on stage, but this is why you bring up about 20 volunteers and then weed out the fakers.
It's more far more serious than a "staged act" effect.
as for the question originally posted, I'd highly recommend you find a mentor nearby that you can watch perform and learn from. You can read books all day, but there is nothing quite like learning first hand and gaining excellent tips and knowledge from those who've been doing it for a while. They can supply you with very helpful tips and some very serious do's and don'ts. It's also nice because they can elaborate on why the do simple, yet things they do, and how they do it.
If you can't find a mentor, I'd recommend the "Little Black Book of Stage Hypnosis Secrets". It's a very good place to start and you can get it for about $15-$20http://www.hypnobilly.com