You show 2 jumbo sized cards . One of the card is face up (say 5 of hearts) and the other is face down. You turn around the two cards to show the other card face up (say 2 of diamonds) and the 5 of hearts is face down now. You then mix up the 2 cards and ask the spectator which one is face down , let's assume he says 5 of hearts. But you turn around the card to show that it's 2 of diamonds. This can be repeated over and over again.
Set of One jumbo double back card and one jumbo double face card