I found Giobbis advice in Card College 5 (chapter 64: Cardmans Humor) very useful. He talks about "general considerations on the use of jokes and lines," it's worth checking out.

Shahin

DPE666 writes: Statement: "I know how you did that!" or "I saw that!"

Answer: "I don't care."

Wonderful! Done in character it's worth a truckload of other lines, culled from Lord knows where and that just don't fit!

I think it's brilliant. (I may be alone, but I still think so!)

Wow. Thanks, Peter.

To: How did you do that?
To a female, I say "If I told you'd I'd have to sleep with you, and my wife gets really upset when I come home and tell her I told the secret. AGAIN!!"
To a male, I say the same thing, but end it with, "And believe me, THAT'S NOT GOING TO HAPPEN!!!"

One I've found effective with my own character is:

Spectator: How did you do that?

Me: (looking perplexed) I don't know, sometimes that just happens!

This also gets me off the hook for repeating a trick. Obviously since I don't know how I did it, I couldn't repeat it at will.

I use a rubber band routine as my typical opener and as prevenative maintenance before any of the typical questions are asked. The beginning effect of me penetrating my thumb with the band is done once. I then state "Most magicians will not do the same thing more than once for fear that someone may catch on, I am different, I don't care." I then repeat the effect allowing the spectator to pull the band threw my thumb. I do not know why but this line gains a chuckle so I use it. I actually don't care for it. If I use this line no one ever asks me to repeat an effect for the rest of my set.

At the end of my routine I say "Many people ask me how I do that...and I ask them 'Can you keep a secret?' Of course they always say yes, so I tell them 'So can I, I'm a magician!' "

If the audience doesn't laugh I say "They never laugh either" which then gains a laugh.
After this I give a mock explanation of my whole routine using broken and restored rubber band which goes like this.

"Since this table is such a great table I will tell you the secret to everything that you have just seen. It takes three skills. (actions matching words) The strength to break a rubber band. The quickness to move faster than superman, where no one can see, so that the band can be moved to the other side of my thumb, link the bands together, make the bands appear to meld threw each other, or your finger (jesturing to the spectator whose finger was also penetrated, part of the routine). And last ,but by no means least, the ability to squeeze the rubber band so firmly that the broken ends permanently fuse back together.

This mock explanation typically prevents eople from asking how it was done as well as any following effects.

If someone states "I know how that is done." to any of my illusions I make eye contact, nod, wink, and say "Wow, Most don't!" and I continue on.

If the guests are the right type and someone asks me to do something again I will say. I'm not a juke box. You do not put a quarter in me, press 5306 and hear Freebird" Being from WV this gains a laugh. I will then ask the guest who requested a repeat if they would like to see something I believe they would like even better. Then I continue on.

Spec: How did you do that?
Magi: A complete answer to that question would involve a complicated discussion of the mechanical aspects of the art of magic coupled with an understanding of the psychological subtleties being employed. But most important to me is the emotional connection which provides the context for our experience, so perhaps I'll just smile and say "Thank you, I'm glad you liked it."

Spec: Can you do that again?
Magi: I certainly hope so -- I have another show tomorrow.

Spec: Do that again.
Magi: Get me a cookie.

Spec: I know that one.
Magi: Do you want a cookie?

Spec: I saw how you did that.
Magi: Yes, but can you see why?

--Dan Harlan

I don’t think you really need a comeback line to “How did you do that?” This is actually a compliment, a way that spectators express amazement. Of course, you could respond with a good natured comedy line like “Very well, I thought!” but the emphasis here is on good natured. I wouldn’t call this a “comeback’ but just a humorous response. More difficult to handle is when the spectator, whether correctly or incorrectly, throws out his guesses as to how the trick was done (sometimes very loudly). This response is pretty much the OPPOSITE of “how did you do that”. It’s a way of saying “I KNOW how you did that; you can’t fool me!”

HOW DID YOU DO THAT?

1.Very Well Thank you
2.Can you keep a secret? So can i

CAN YOU MAKE MY WIFE DISAPPEAR?

1.Just be Yourself
2.I Perform Magic not Miracles

I KNOW HOW YOU DID THAT?
1.But do you know why?

When I do magic for kids, my answer to this question goes something like this:

Them: How did you do that?
Me: Do what?
Them: Pull that brick out of Timmy's soup.
Me: Pull the who out of the what now?
Them: The brick! You just pulled it out of Timmy's soup.
Me: I don't think so. That sounds impossible.
Them: But you just did it!
Me: You have quite an imagination!

Lee--
Goldman wrote the screenplay, too. He told my agent once that the book and the screenplay are the only things he ever wrote for love.
Oh...
And he's writing the Broadway musical, as well.


I generally say, first, "It's a trick," with a Paul Lynde delivery, then move on.
If they insist, and shout out a method, I generally say, "Wow! That's a lot easier than my way! Thanks! Here's another one you'll like," and move on.

Hay try this one,

What’s the difference between a magician and a musician?

A musician takes request!!!

The ruder the funny, but not to much.

I use "very carefully" a lot too.

One of my favorites is: "Money, brains and guts."

A variant:
"If you want to know the truth (lean in confidentially), I think Oswald had an accomplice when he shot Kennedy."

One I've used for children who play the "I know how you did that" card:

"Good. Let's keep it our little secret." (wink and go immediately into the next trick)

I cannot remember where I heard this now, but it's advice that I think holds true:- If you get involved in a battle of wits with a true heckler you have to be funny every single time. After all, you are the Pro. If the heckler gets a bigger laugh than you just once then you've lost the whole battle.

Luckily I've not been in that position (and wouldn't want to rely on my own wits to survive such an ordeal).

I saw this in other forums.
"Can you do it again?"
"You know, man can't do it again right after, I need a rest."

I don't have this type of request after a trick. I more often get this question on tricks I didn't perform: "I know that you cannot tell me how you did this, but tell me how the cut a woman in two". Then I answer "well that's stage magic and I never did any". The few times I got the question about an effect I had performed, was more generally at the end of the show. Since all of my effects involve the audience and at least one spectator, I should probably answer: "Oh it worked better than ever tonight, probably because the entire audience was so brilliant." If they insist: "well you should know: we did it together" if they keep on insisting look at the person with a smile in your eyes and tongue in cheek "Frankly? I can trigger it, but I really don't know how it works".

For the "can you do it again": "Not right now. Maybe later. right now I'd like to share something even more incredible with you. Just keep on helping me on this one. You really did great on the previous one."

At one time I was fascinated by "double-talk". Never went anywhere with it, but I still bust it out once in a while to relieve the boredom. Like when someone asks, "How did you do that?", I'll reply, very slowly and clearly, "I had the fleetisway behind my spurl, which I then put on the arbigon."

Or sometimes I'll reply to a "how did you do that" question with, "Well, I shouldn't be revealing this, but it's done with mirrors."