Place a deck of cards on the table. A spectator thinks of a number between 11 and 19. Lets say that the spectator thinks of the number 12. He counts down to that number in the deck by dealing a card for each number. When he reaches 12, you tell him that you really want to mix things up. Have him take the stack that he counted onto the table and place it back onto the top of the deck. And then ask him to add the two digits in his number (1+2=3). The spectator then counts three cards from the top of the deck and turns over the third card.
Lets say that the card is the two of clubs. You point to an envelope that has been in full view the entire time and ask the spectator to open it. Inside, a note says You will pick the two of clubs. You have successfully predicted a card that your spectator selected.