When the winners find the business cards they get to come up on stage to redeem their prizes, but there is usually a catch. I make up some funny questions to ask them. They get the prize no matter how they answer.
If you are pressed for time, you can hand the prizes to people in the first row and have them hand them one-by-one back to the winners. This gets many people involved physically because they have to handle the prize. (It's not a bad sales technique either. You might use your product for the prize if you want the other audience members to touch it and want one too.)
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