Do As I Do With Two Decks

     This effect is from a book by Bill Tarr, called 101 Easy Magic Tricks You Can Do.  It is on my recommended reading list for all Magicians.  

The effect begins with the Magician having a spectator choose one of two decks of cards.  Once chosen, give them the option of changing.  Then explain you want them to do exactly as you do.  You may begin with a few simple shuffles and flourishes (they may not keep up).  

Once both decks have been completely shuffled, glance at your bottom card (this is your key card).  Switch decks with the spectator, and have them select any card in the middle of the deck, you will do the same.  Have them show their selected card to the rest of the audience.  Place both cards on the top of their respective decks, and cut the deck.  This puts the key card on top of the spectators selected card, in their deck.  

Now switch decks with them again.  Have them go through their deck and pick out their selected card, but tell them not to reveal it to you yet and that you are going to do the same.  As you go through the deck you now have, you select their card based on the key card (your selected card doesn't matter).  The key card will be to the left of their selected card as you go thru the deck from top to bottom. Each of you places your selected card on the table face down On the count of three you both show your selected cards to be the exact same.