Think of it this way: How would you character turn someone down?
I have avoided this by making sure that I can pretty much always perform all of my material at all times - but for those few things that do require a set up I use pretty much the same approach as the previous guys.
I also have the advantage of being an esoteric performer so it's completely legit to say, "The energies are right at the moment.", "That's something I have to prepare for." Or something like that. I am also a fan of basically saying, "Oh you liked that one? Here, watch" and then just doing whatever I feel like doing. Usually what they're really asking is to see you perform something in general - they usually won't mind if you don't do the exact trick they mean.
This is also a good reason to learn multiple methods for the same effect. Note: Effect, not method or trick. I have several different ways I can do PK touches - everything from gimmicked to impromptu and everyone that sees me do it gives pretty much the same report on what I did. Same with when I used to do metal bending. Multiple methods, all engineered to look more or less the same, but some gimmicked, some impromptu.