With many volunteers, one thing I do is to take the sting out of any trick I might do and turn the laugh on myself.
By making myself the butt of the joke (something I have spent my life being anyway, so I'm actually playing to type!), the volunteers all seem more at home and safe.
Example - card to forehead. This trick is quite surprising and gets a huge laugh - but some folks take it as an insult to their intelligence, so I turn the laugh with a line and a gesture.
"Hey, don't feel too bad! Think of how I feel having to do THIS for a living!" and point at the card, while mugging a grimace.
By putting the joke on me, instead of them, and then thanking them for their help and for being a good sport, the concerns of the majority of the crowd seems to dissipate.
Being a natural-born buffoon helps, of course...!
Lee Darrow, C.Ht. http://wwwleedarrow.com