Read my tutorial on stripper decks or the tutorials on other tricks involving them as they work differently than regular decks and allow you to easily locate a card whenever you want.
Ask your spectator to choose a card.
As she looks at it, rotate the deck 180 degrees. Have her insert the card back inside. This will cause her card to slightly protrude.
Place the deck behind your back and explain how you plan to find her card. In actuality you are rolling her card up into a small piece. Hold this piece behind the deck, hidden from view.
Bring the deck around and remove the bottom card, asking if it is hers. She will say it is not. Repeat this step a few more times, each time pretending to reveal her card through a sneeze or some other way that requires the deck to be close to your mouth.
On the final time, slide her folded card into your mouth. When she says no, act dejected and sad because you seemingly failed the trick.
Suddenly, begin coughing. Make sure this feels real and not part of the show. Cough up the card and unfold it. She will be grossed out but insanely amazed.