1. "Do it again!" "Alright, but this time I'm going to try something a bit different"...then proceed into something similar but not the same. Either they want to catch you out the second time, or if they are genuinely interested in seeing it performed again they won't be as impressed because they know the punchline. You wouldn't retell a joke straight away would you?
2. "Do another!" I'm learning to say "No" to this.
3. "How did you do it, tell me pleaaaaaaaaaaaaseeeeeeee...!" "No" although, during some of my mentalist routines I'll talk about techniques that I am
apparently using like body language or subliminal cues which tends to keep people from asking that question. Tends to, but not always.
4. "I know how you did it." "That's interesting I like to keep that sort of thing private

(OK I know it's a terrible joke but I have actually used that, it got a laugh and shut the guy up) Either that or "Excellent, but don't spoil it for the others." Sometimes if I'm feeling daring (although being a bit of a newbie that isn't very often) I'll use them for the next trick, "Is that so? Well then you'll be perfect for what I'd like to try next." They don't act as cocky then. Ha.
5. "That's so simple, you did it by using ......." No matter if they are right or wrong I just say "Well no, but that way sounds much simpler. I'll try that next time."