Steve, this doesn't have to be such a 'quick' trick. You can first show them the deck, spread them out, show that they are all facing up. Ask them to think of a card, no, just think of one, you ain't gotta pick it, just think of one.
Was it the Ace of Spades? No?
How about the 2 of clubs? No?
Could it be the 6 of diamonds? Wrong again?
Well, lemme see - I'm getting a color - RED - Is it a red card? No? OK
Is the card higher than a 6?
Now when they start to tumble to the fact that you are narrowing it down stop and look at them. You're absolutely right, out of curiousity though, what card was it?
When they tell you show them their card is upside down in the deck.
Heck, I reckon I should'a looked here first, huh?