Deal 3 cards across 7 times so that you have 3 piles of 7 cards each. Put the rest of the deck aside. Have a spectator look at one of the 3 piles and memorize a card in it. Have him (or her) put the pile on top of one of the other two piles and then put the other pile on top of that, so that his pile is sandwiched between the other two.
Deal the 21 cards again into 3 piles of 7 cards each and ask the spectator to guess which pile of cards contains his card. Have him verify his guess. If he guessed wrong, have him guess again until he has found the pile with his chosen card. Again have him reassemble the pack so that his pile is between the other two. Deal the 21 cards once more into 3 piles and have him find the pile with his card. But this time, have him give you the pile.
Spell out the sentence "THIS*IS*THE*CARD*YOU*TOOK*". As you say each letter, move a card from the top of the pile to the bottom. As you complete each word (*) , deal the next card onto the table. When you are left with only one card in your hand, it will be the spectator's card.