Ask a spectator to shuffle cards. When he/she hands them back to you note the bottom card. That is "their card." Start by fanning the cards out. Run your finger across them and ask the spectator when to stop. When he/she says to stop, put your thumb on that point.
While your thumb is on that point, use your other fingers to slowly work the bottom card under your thumb. As you pull the cards off the top of the deck with your thumb, slide the bottom card under the other cards and pull them off the deck and show the spectator his/ her card. This is where you take over. Since you already know the card you can have the spectator cut, shuffle, etc.
My favorite ending:
Start flipping the cards from the top of the deck over onto the table. When you flip their card say, "I'll bet you a million bucks that the next card I flip over will be yours!" They'll think you messed up because their card is already flipped over on the table. If they know about card tricks they will think that you used their card as a key card. When they say, "Make it two million and you're on!" flip their card, which is already on the table, face down. They'll be dumbfounded.