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Take a pencil and make a slight 'line' down the long side of the deck, not in the middle of the side though. Now, fan the deck and ask a spectator to take a card, any card, remember it, and stick it back in the deck at any point.
While the spectator has the card in his/her hand, turn the deck around so the side with the mark is now on the opposite side, assuming the spectator is going to put the card back in the deck the same way he removed it.
Once pushed back in the deck, the magician can now shuffle the deck thoroughly before revealing the selected card... which should be easily recognized as it's the only card on one side with the pencil mark.