Whenever someone asks me how I did a certain magic trick, I just smile and say that it's by magic.
Of course some people would say that there is trickery or it's just that my hands are fast. When I hear that I just smile back at them.
Well an annoying situation might be when someone says how you do the trick although more often than not that person really doesn't know how the trick is done. I've actually heard someone once when I've performed said that it's more fun when he solves the trick.
Ok I'll leave you to your happiness but as far as I know you may know ho it is done but surely you'll never be able to do it. Also I've heard that same guy explained to the person on his side how it is done but as I was listening to him I just laughed from the inside because what he was saying was really a different method for a trick. I was doing an Ambitious Card routine at that time but when he saw me placed the chosen card face up on the center and having it come back to the top, really he just stood there silent and maybe embarassed that he tried to expose me.
So to all magicians out there, don't be afraid of those heclers who says they know how to do a trick. Usually what I do is show them a "sucker trick" wherein a trick might seem to go wrong only to find out in the end that the trick is working fine all along.
Also I try not to present a challenge for this "know it all" type of people, doing this might make them more observant and even to nosey that they will roam around as you do your trick, viewing some angles that might give away your trick.
To sum it all, just remember that when dealing with those know it all hecklers just remember that no matter what they say that they know the trick, they can never do it because nothing equates a magicians dedication and hardwork that has been put into a trick.