Dan Harlan's "Birthday Suit"
You offer someone a brief astrological reading during which you reveal that you have predicted that person's birthday.  
All you need to do is ask a spectator his/her birthday.  After the spectator answers, you can tell them their astorlogical sign.  Then you tell them that before you came to this event you turned over three cards in the deck for a person that you would meet and that you now believe that person is him/her.    
As you spread through the cards in the deck you find three that are face up.  When the backs of these cards are revealed, they are seen to have the spectator's birth date written on them!  
Easy to do.