Seb's Bottom Trick
Keep the cards in a single pile. Be sure to find some way to gain knowledge of the bottom card (shuffling, dropping a card, etc.) Hold the deck in your left hand, in a normal dealing position. Put your right hand on the deck so that your right fingers are on top, thumb on bottom, as if you were going to pull the whole deck back out of your left hand.
Your right fingers begin to slide cards back, one by one, and you tell the spectator to say 'stop' at any
time. When they say 'stop', use your right hand to pull off the cards that were moved. At the same time, your left thumb presses on the bottom card. As you slide the top cards away from the deck, the bottom card will come with them. (This works best if you pull the bottom card out, little by little while you go through the top cards.) Hold your right hand up so the spectator can see the bottom card of the group, but you can't.
Now announce the card that was on the bottom. To make it more mystical say something about how you can tell it isn't a black card by the expression on their face, or that you can tell it's a Heart by the way their fingers wiggle. Or come up with something better. (That shouldn't be too hard.)
Editor's note: finding out the bottom card isn't hard, especially if you are performing more than one card trick. Handling the cards, both during and between the tricks, gives you lots of chances to see the bottom card without making it obvious.