2CM is a great practice for this. Its either because they lost focus of attention cannot hold much longer, they are not in the zone. Today i performed a quick Four Ace routine, i had him hold one ace (i forget which) face down, infact that ace was switched for the opposite suit, he tried turning it over. I jus held out my palm face down to direct his attention back.
Normally during things like these i would force their direction back towards what i am doing, because for me once they are linked, i chain combo them with my army of 52. While i was at school performing for people in my Year, it was hard because i could be easily made fool out of by other Year mates, most of which i dont talk to or seem inferior to, but i try to develope a relationship with them through performance. I treat them as if i am giving them a gift, so i ask questions to assure they trust me. Things like "Am i going to fast?", "Here, trust me man, hold this card like that in your hand". So most times people i perform for come merrily, go happily.
Its not good, in the middle of a performance to go telling someone to not turn over the card when they feel, or is about to turn them over, it ruins the performance. "So, you have chosen a - hey don't do that!". I try mostly with indirect suggestions as to not be foolish by turning the card over.