You show a stack of Poker sized cards. The top card has a picture of a genie with his hands empty. The spectator signs the top card. The spectator then selects a card and magically the card appears in the Genie's hands. The spectator can validate his signature at the bottom of the card. The rest of the cards can be handed out to show that they all have a genii printed on it with empty hands.
The cards are specially treated so that you can write on them with a dry erase marker and wipe it clean.