Try this:
Actually do something similar, but do it with a different method, so that if your spectator is looking for the method, he or she will have no idea what to look for. Obviously, this only works for certain things, like the Ambitious Card, or some sandwich effect.
If they ask to see the effect again, just do something bigger that will completely blow them away, something preferably long enough that it would be tedious for them to want to see it again.
If they insist, just shrug and say "What? Do I look like your freakin' magic clown?"
Works every time.