Secret:

First, let your friend shuffle the deck. Shuffling always adds to an already good card trick. Fan the cards out in the best way that you know how, and ask your friend to select any card from anywhere in the entire deck. As they take it out, square up the deck. Cut the deck somewhere in the middle, taking the top half with your right hand. Tell them to look at their card and remember it.

Outstretch your left hand, and have them set the card down on that half of the deck. While they are doing this, or while they were looking at their card, whichever is less obvious, glimpse the card on the bottom of the pack in your right hand...and remember it! Instruct them to place their card face down on that packet in your left hand. Drop the right hand-packet on top of the left-hand packet. Once again, hand the deck to your friend and tell them to cut the deck and complete the cut. They can do this as many times as they want, but only single cuts at a time. (This will not affect the order of the cards.) Take the deck back.
What you have just done is placed a key card right above their card. You don't know the identity of their card, but since you know the card right above it, it is almost as good! Now, there are several finishes to this trick. All of your work is practically done, and here is where your acting ability is put to use. Make these finishes as dramatic as possible, act as if you yourself are amazed, and what an incredible sleight-of-hand artist you are.