Bring the chosen card to the bottom of the deck. (You can use false shuffle: overhand shuffle control). Then set the last card on the bottom (the chosen card) of the deck a bit emerge toward you. Hold the deck of cards on the edge with your thumb and index finger. Ask your audience to slap the deck at another edge. source: http://www.cardtrickcorner.com/slaptechnique.htm

When someone picked a card, let them put it on top of the pack. Take about 1/3 of the pack with the selected card and put it on the bottom of the pack. Now take the entire pack of cards and slam it on a flat surface without applying pressure (release it before it slams). You'll notice one card popping out, the selected one. This is simply because there is a slight bit of air on top and on the bottom of the card while the other cards stick together. It takes some practice but it should work just fine and it's quite entertaining when performed correctly. Good luck!