Shin Lim's THE SWITCH is the cleanest way to switch a folded card for a spectators signed card. It looks so good that people have been accusing Shin of using camera tricks. Having seen it performed live, we can assure you this is completely legit.
You place a mystery card on a table, in someone's hand, in a clear box, well... anywhere. This is done in full view of the audience. You then have a another spectator select a random card and sign it. On your command you cause their sign car to vanish completely from the deck. The spectator is asked to open her hands, the mystery card that the spectators has been holding the entire time is opened. It is the signed selection!
THE SWITCH is the cleanest way to switch a card because there are no boxes, no envelopes, etc. Words cannot describe how clean the switch actually is. Check out the trailer.
If you have seen Shin's 52 Shades of Red, you'll get an idea of the kind of thinking that has gone into Shin's recent work. The method for The Switch is along similar lines and is surprisingly very practical. You'll need to make it up yourself with the gimmicks provided, but that won't take longer than an hour.