A great little trick to keep in your wallet.
 
Ask your spectator to think of any playing card.  Ask him to look through the plastic cards and keep the ones with his thought of card on - handing you the other ones. 
You immediately know which playing card he is thinking of! 
 
The cards come in a handy vinyl wallet.  They are 13.5cm x 7cm and all 8 cards together are only 2mm thick - ideal for keeping in your wallet or jacket pocket.  Why not get 2 sets so you are always ready to amaze someone?