That's quite a good question, it depends of the trick/routine you're performing for instance if you are doing an ambitious card you won't have any trouble at all but if that's not the case and you just can't repeat the trick for any reason, you can say something like this: "I'll show you a better one" "If that's the case, I'll wait for your call -hands business card-." If you want to joke around a little "Exactly what do you want to see again? *they describe the effect* But that's impossible, I wish I could do that." or simply say "No" you have the right to.
Cheers!