well, although i practice till i can't get it wrong (kudos to jamesjay), i still get the heklers who think they know how its done and yell out random way off ways.
there is only one solution...turn the humiliation on them, but politely. stop if they are getting offended.
for instance, i was doing some card tricks for some peope at a bar. I finished with my impromptu signed card to wallet, and the spectator gets to take out the card . Now the wallet had been on the table the whole time.
the heckler (whos card it was) said "I saw you slide that in ages ago" "really" i said. so i took his signed, folded card and got my lighter and burnt it until it was ash, then rubbed the ash on my wallet and then said to him. "Open the wallet then genius", there was his card again folded and in the coin compartment, everyone lauughed at him.
i said "Explain that one then" i did not get bothered again for the rest of the night, and best of all, he was not offended, which is the key thing, you don't want to be offending your specs especially if he is friends with your boss.
If they still bother me, i steal their watch. And give it back later of course.
PS. Although i can't tell you how i do my tricks, the message is there, use your skills, which come after months of practice and alterations, to ensure that you are not the one humiliated in YOUR OWN PERFORMANCE.
And as jamesjay said, proffesionals practice until they can't do it wrong, this is one of the most true thigns i have read on this forum.
Before every performance, piad or not paid, i practice my routine again and again and again.
If i can't do my routine 10 times in a row PERFECTLY, then i will cut out the part that is holding me back and save it for another day.
and remeber, misdirection is by far the most powerful tool when it comes to hecklers who "burn your hands".