A volunteer memorizes the cards on either side of a card she hides upside down in the deck.
You produce both of those cards from your pocket - without even looking at the cards.
Take 20 cards plus a joker from a deck of cards. Set aside the rest of the deck.
Give the deck to your volunteer and ask her to shuffle it a couple of times.
Have her slide the joker into the deck anywhere, face up, then fan the cards out and note which two cards are flanking the joker.
Instruct the volunteer to return the deck to you, squared up so you can't see where the joker is.
Deal the cards into two piles, beginning on the left. The final card should go on the left stack.
Pick up the right pile and put it on top of the left pile.
Do this again, putting the right stack on top of the left.
Hold the stack in your left hand.
Say, "We don't need this joker anymore," and thumb through the face-down cards until you find it.
Pull it out and toss it aside, keeping the cards above it in your right hand. Casually and quickly put the two halves of the deck together, slipping the top half underneath the bottom half.
Have the volunteer cut the deck.
Put the upper half in your right pocket, with the cards facing toward you.
Put the lower half in your left pocket, with the cards facing away from you.
Thumb off the four cards furthest from you and pull the fifth one in each set. These will be the two cards that were on each side of the joker.