i like to put the card beneath their shoe for tricks like that or clasped Between their hands, they rarely look at it, i don't reccomend letting them hold the card beween just their thumb and pinky AND NEVER EVER LET HECKLERS hold the cards like that!
Besides posting in a topic nearly 2 years old. I disagree with most of your advice.
If you would learn some control you wouldnt have to put them under their shoes. That just sounds rather pittiful and lack of effort.
You really have alot to learn about crowd control, assistant control and just about hecklers in general.
Theres just no reason to have them put the card under their foot or under anything but their hand, enless they are just overly wet or dirty.
The between the hands is better than the shoes.
Learn control and you should have no problems.