Show the spectator three cards with psychic symbols on them.  Explain that you as the magician can mentally force the spectator to choose the card of your choice.  Then ask the spectator to point to one card as you concentrate.  When the spectator has made a choice, show that your prediction was correct by showing that their chosen card has already been written down on another card which you then display.  Very easy to do and you don't really need to have psychic powers.