in many cases it depends on the trick,
here are some general ones tho:

sometimes il say: i dont know you tell me, you were holding the freaking card all the time(or you were the one shufeling the cards, or you sent me the image of youre card and so on, you get the idea)


or here is another one:
normaly i wouldnt tell you.... and since this is a normal situation iw wont
or
well, id tell you but (pause) i wont
or
i know its weird isnt it, know whats even stranger?, this thing right here(move on to the next trick, they will forget they ever asked)

or just act realy surprised and freaked out, kind of like they would except youre doing it first: OMG how did that card get here
or
wait a minute i dont get it... did you do something tricky??? how did that happen?? il have to keep my eye on you

all depends on the crowd, you can come up with anythig on the spot depending on the situation, and teh trick

but my goas is for them to not even ask, and just go with it, and accept the fact that its happening, and laugh, and be entertained

I don't know, how did I?

I forget or If I told you I'd have to kill you

A smirk and a shrug always worked for me.

Here is a few of my answers:

How'd you do that?

1. Oh, that particular one...was magic. All the rest were just tricks.
2. Years of practice.
3. By accident. (usually a fire trick)

...or before they even ask

4. "don't tell anybody how you did that"

i like ur avatar =]

n i usually say "what did i do? i didnt see anything, did u?"

I tell them stuff like:

- Magic *waving my hands infront of them*

- I have no idea actually!

- If I told you, I'm afraid I'd have to kill you. (Put on your best James Bond-Face when telling them that.)

Why do you even have to respond with anything?

Usually I don't respond to them and just continue with the routine.

If it is the end of a routine, sometimes it is best to have something witty or funny to say as opposed to ignoring the question. But you're right. If a spectator is asking this during the routine, it is best to continue the routine. But seriously, there may be some timing issues if the spectator is getting off that type of question (expecting an answer) during the routine.

The reason why I usually ignore this question, is because most of the times this question is being asked, it is asked as sort of an amazement reaction. They don't expect you to actually inform them how a certain effect was accomplished, but they are simply showing you their amazement. (usually it is easy to tell by their voice tone whether they actually mean the question or not).
Of course, there are spectators who actually do mean it. In that case, I agree, use a cocky and funny response. 'Pretty well I think' works great for me. But usually I just don't respond.

FOOL: how did you do that?

ME: you tell me... you saw it happen

If I do a rising card, I would say, "You picked a card, put it on top, I covered it with about 12 or so cards, and I made it rise."

basically, explain the performance.

Guy/Girl: How did you do that?

Me: What?

Guy/Girl: (Describes it)

Me: darn, I missed it...wish I saw it for once...

---

Guy/Girl: How did you do that?

Me: I read the bible...that book tells you everything.

(sadly I sad that once and some kid came back to me two weeks later complaining about how he read the bible and theres not a single magic trick in it)

That's kinda bad. That's why I don't make references to anything religious while I do magic.

i dont know im still trying to figure that out myself.