Hand a full deck of cards to a volunteer to shuffle. (Make sure it's a FULL deck.) Ask the volunteer to deal the deck into two piles. Have them choose one of the piles and remember the bottom card. Tell the volunteer to show the bottom card to the audience (anyone else who's there,) but not you. Ask them to place the pile containing the bottom card on top of the other pile. Then ask the volunteer to deal the deck into four piles from left to right.
Pick up each pile and ask the volunteer if their card is in that pile. As you ask, reassemble the deck, making sure the pile with their card goes on the very top. Give the deck to the volunteer and have them deal the cards, one at a time face down on the table, spelling out the word "PRESTO". Have them turn the next card over. This is their chosen card!