I've been hitting Fast and Loose, hard and fast at the moment and I've noticed a few different types of marks whose way of thinking tends to lessen the impact of the routine.

For example, some people make a big deal about not caring if they win or lose, others waste too much time choosing a loop and others always exclaim "oh, I bet a lose' no matter where they pick and what they bet.

Its usually an act so they don't feel stupid if they lose.

I haven't seen too many of the "not caring" folk, but a lot of the other two. Even when the prize was a worthless souvenir, they would obsess over their decision to the point of others losing interest and walking away. For the last group, I like Whit's shell game line "That's a defeatist attitude!" and keep the pace moving by repeating the "proving" moves once again. If I keep the pace brisk nobody seemed to think ahead enough to challenge that each "proof" destroyed the evidence, and the subsequent toss could be crooked once again. If so, I was ready to "prove" that one too, and I think it was my attitude toward making certain that I was "playing fair" made them finally soften up and play along.

Go see the suggestions on Whit Haydn's "Scoundrels Forum" http://www.whithaydn.com/smf/index.php
There's enough there to put any kind of spin you want on your approach.