Now - One of the most simple tricks to do, but it look really good it done right.
grab about 10 cards - any cards.
Show the top one to the audience, and say it'll change.
Also tell them to watch the top card - so you can't cheat *cough*
Make sure the deck is completely straight, other wise you'll see the other card behind it.
Put four of your fingers on top (Could be 3, but works best with 4) and your thumb on the bottom.
If you do this fast enough, they cant see your thumb carrying along the other card underneath.
Once you've pulled it out, your thumb should be holding the bottom card in front of the actual top card!

Tips: Do not hold the deck tightly, other wise you wont pull it out.
It might take some practice, but you'll get it perfect before you know it!