Today many people think that when seing a magician perform the POINT of the whole thing is for them to try and figure out how it is done. It is up to you to teach them otherwise. My guess is, 9 out of 10 times the audience member doesn't even realize he/she is being very rude to you. True that comment makes your trick very much less impressive, no matter if they are right or wrong about it. However, it is merely a way to respond to your magic, like the phrase "How did you do that?". Most of the time, this is just a reaction to your (great) magic, more than an actual question. The audience member is not trying to be better than you, but simply thinks he/she did what he/she is supposed to do, when suggesting how it's done.
NEVER use your magic to make anyone look like an idiot! You BOTH end up looking bad, and your audience will feel the akward moment! There are times when you can use the "heckler" to your benefit, but be VERY certain that you can work it to your advantage. Remember: Anything that doesn't add, detracts!
I was doing a show in a school once with relatively young kids. It was a big school, and it was a traditional school "fair". I had to do the show three times that night, so that people could go around and see what the kids had made, and then see the magic show when they felt like it. This meant that all the teachers were busy and noone were supervising the kids watching the show. One kid in particular had NO manners what so ever. He acted like he was the one to watch, and with all my experience I REALLY tried to get the best out of it. It was impossible, and with no teachers there to help, in the end I just had to ask him to leave in order for me to finish the show. Some parents came to thank me after the show, and they had been video taping. I told them I was glad to hear it, and made a comment about it being a shame I had to tell that kid to leave when they were filming. The woman immediately said: "OH no, thank God you did that! I thought you would have thrown him out much sooner!"
So was it my job to throw that kid out? You could say no, because a teacher or parent should have been there to sort the kid out. Did I do the right thing? DEFINATELY! You are not responsible for the actions of your audience. You are however responsible for the entertainment of the people who come to see you and enjoy it! My only regret now is that I didn't stand my ground much sooner, cause I already knew on my second nice try to silence him that it was a dead end.
You are hired to entertain, and it is the person that hired you that has the job of keeping your audience at an acceptable behaviour. If they don't help you in a tight situation, simply state to the full audience in a quiet voice that you are sorry, but you cannot continue your show until this person keeps quiet or leaves the room.
Also remember that you can always ask the person that hires you for certain settings. Don't be afraid to ask a certain distance to the audience, if you know you are doing angle-sensitive magic.
Now, everytime you can "dodge the bullet" it really shows that you are confident in your material and as a performer, thus it will add to your performance. That's why I like some of the suggestions posted in this thread like: "You know how it's done? Well don't tell anyone, they'll think you're in on it!" or "You know how it's done? Why so do I! What are the odds of that?" To the question "How'd you do that?" you could use a joke like "...Pretty well dont you think?" or simply say "It's pretty neat isn't it?" and go on with your tricks.
Everytime you can make a joke that neutralizes the threat and makes you look stronger, by all means do it.
As a finishing thought I would still like to hear you all come up with the perfect sentence to close a challenging comment. I think it is VERY good to be prepared for difficult situations, and in general to go over what could make your performance go bad. Now don't go on stage thinking about how this and this can fail. Then you'll set yourself up to do that very thing. But preparing what to do if you drop a coin... that's just common sence and proves that you are really a professional.
Give me that perfect sentence that tells the dude "Hey man you're kinda making me look bad here, and you're demining all my hard work!" without him feeling offended or the rest of the audience taking notice. Another thought could be, would it just be better to ignore it right now, and go on with the next trick? Also you could try to patter or routine in a way that teaches your performance how to watch and enjoy magic, instead of the misconception of trying to figure it out.
Thank you for reading my thoughts. You may not agree with all of it, but that's fine. As long as I made you think I'm happy.
Good luck in all of your future performing!