Whenever performing stigmata, I really like to have them take a firm grip to my arm as a firm grip produces visible finger imprints from which the card selection will appear. I've had people just kind of place their hand around my arm and while the end result is the same, i'm partial to the "capillaries moving under my arm to form your card" style of revelation- i've found that it's more convincing for one reason or another rather than just having something pop onto your arm. To ensure a firm grip, whenever i get a dead fish grip from a spectator, i say "Oh common, hold it tighter... it's a magic trick, you're not going to break me" Needless to say, during the first few weeks of me learning how this trick was done, I found myself in a position where the guy (about 18 or so) didn't have a nice enough grip to produce the finger marks. I said my little quip and to my surprise (and later dismay) the guy puts my arm in some kind of Vulcan Death Grip. Almost immediately my hand begins to get pins and needles and I realize that before my nailbeds turn blue, i should just reveal the card. He was absolutely blown away with the whole trick and i walked home. Within the hour, i was looking at my arm and noticed that the kid had actually grabbed so tight that he blew a few capillaries and my arm was black and blue where he was holding for a good 2 days. I've since changed my patter...
In regards to having the wrong card appear on your arm, if you watch the dvd, the little patter dispersed throughout the trick serves (for me at least) as a check for the whole operation... (i.e. "Now if i told you that your card was black [they nod] that would be slightly impressive but that's only a 50/50 shot so we'll take it one further. if i told you that your card was a spade [they smile and nod] that would be more impressive but again, that's still just a one in 4. But now i want you to only think of the number of your card, this is hard because i have 13 to pick from" or something along those lines)
Now if when you say that you feel their card being black (and i usually do this "through vibrations" while they're already holding my arm) and you're wrong, well that's when you know that you messed up and you resort to damage control. "Hmmm... I must be a touch off this morning, here, lets try something different" and then you can go into anything (although if you wanted to know what went wrong with the trick/ what card they picked, "Do as I do" is a great way to solve both the fact that you'd like to know what card they picked and the fact that now you have to do some form of magic, you kinda set yourself up as an illusionist/ magician/ cardician/ etc... already)
Again, not to get into any reveals or anything, practicing the 'first step' over and over and over and over again will ensure that you don't have that problem again. I admit I did have this problem once before but it was due to my own ineptitude:
I decided to do an ambitious card routine with a true free selection and never having looked the card that they chose during performance. I figured that towards the end of the "set" i would have them place their card in the middle of the deck, do a Dai Vernon in jog/back jog deal, peak the card and go into stigmata. When I went to form the break (please read Inner Secrets [chapter 2?] by Vernon for the full out explanation), I went did a bottom control rather than the top control that I was thinking of (when you slide cut the deck, depending on how you made the break, this'll happen). So after a more than average ambitious routine (yeah man, i pulled out all the stops), I crashed and burned when a 7<> showed up instead of the spectator selected 5 of clubs.
I'm not one to admit defeat so again, the do as I do saved me (as did the fact that I am a complete magic geek and have 2 decks with me most if not all of the time "just in case"). At the end of the trick, she turned over my 7of diamonds on the top of her deck after shuffling and i 'found' her 5 of clubs so i tied the whole thing back together.
Knowing a nice backwards and forwards repertoire of magic tricks (cards, levitations, what have you) helps a lot with screwups like the aforementioned. When you get comfortable with fundamentals and realize that production is what you make of it, you can save yourself from most "Uh ohs" Hecklers or hand burners are a different story but even still, knowing methods backwards and forwards means you've looked at yourself performing it ad nauseum; from every one of the 190 degrees that your audience could potentially be surrounding you in a mirror etc... If you see someone who is burning your hands, do something that doesn't involve anything that will be seen from where he's standing. Include a heckler into the trick- so far into the trick that he won't foresee what's going to happen next. I think the best thing i've learned however (as everything i've stated so far is kinda standard) is never be forced into performing a trick if you're not mentally prepared to produce one. Don't get forced into doing something slipshot at a nacky angle because it's obviously more impressive to show one trick that can't be explained than 3 that they caught you on in regards to one aspect of the trick or another.
