The "Do It Again" question really is an indication that you are presenting magic as a challenge. The spectators are really saying, I didn't figure it out the first time, can you do it again so I can try to figure it out?" Much of that is a result of people being conditioned that magic is a game -- magicians try to fool them, they try to figure it out. Unfortunately, in that game, someone is a loser. But part of it is the magician's presentation which focuses solely on the "trick." If your goal is to trick or fool the audience, then you may be to blame for the "do it again." Focus on entertaining.
"I'm still working on that one. You be the first to know... after my wife."
I've never been asked to "do it again", but have been asked if I can saw someone in half. I use one of two replies: "Of course, sawing in half is the easy part, its the putting them back together that is the hard part" or "I tried it once on my wife's half-sister, it didn't go well."