This was posted:
Here's what I would do... work out a trick which misleads the spectator into thinking they know the trick, but they are wrong. For example, if you're making a small object vanish, find a trick where you can "finish clean" (nothing palmed etc.), and pretend that you are palming something. Even try to be slightly clumsy about it so they are 110% sure that you are hiding something in your hand. When your spectators say something obnoxious like "What you got in yer left hand, sonny???" you say "What, here?" and slowwwwwly turn your hand over and spread your fingers to reveal that they are dead wrong. Of course, even if they politely clap, be sure to reveal that hand no matter what! Usually that's when the trick really hits home.
I agree here. Do something like the rub-a-dub vanish which is a four or five second trick (well, really its a move, but you can make it a trick). When you lift your hand, you show the card is gone but keep your hand face down and as though you are palming a card (NB make sure you have no sleeves or rolled them up). Perhaps they will blurt out "it's in the hand", but it's not necessary.
At this point you have placed a card on the table/floor, made it disappear and show this by lifting your hand from the surface (not turning it over). Now after a few seconds put your hand back onto the surface and start rubbing again. As your rubbing, allow your fingers to spread and then turn your hand over when you're finished rubbing. DO NOT rush this - rub slowly and smoothly.
The audience will be somewhat confused at this point. This is called social cryptoamnesia. In lay terms, they audience has come to the conclusion that there is a card in your hand (because they figured this out themselves - do not tell them this!) because it is awkwardly palming. Anyone who thought that the card was never there completely forgets this theory and picks up the new one of 'the card is just in his hand'. When you make this card disappear the effect is amazing.
On the other hand you could just do a trick that nobody can figure out. You could either do a triumph effect, or a sucker effect (such as showing their card on the bottom of the deck, trying to determine whether this is their card through mind reading, and then gliding this card. Then reveal you knew their card all along by some means).