The trick: have your mark pick a card. Shuffle the deck, and have them cut the deck into three piles. Show them the bottom card on each pile and put it on the table face down. Tell them that they did not cut to their card. The bottom card on the 3rd pile will be their card, but don't let them tell you that it is. Ask them to pick a number between 1 and ten. Bet them that their card will be that number of cards from the bottom of the deck. Since they know that their card is on the table and not in the deckl, they'll take the bet. Then count out cards and show them that the card that they picked has somehow ended up back in the deck.
The solution: After your mark puts the card back on top, use an overhand shuffle to control the card to the bottom of the deck. After they've cut the cards and you've shown them their card on the bottom, use a move called the glide. With your pinkie or ring finger, slide the bottom card back a quarter inch. Then pull out the second card from the bottom an put it face down on the table. Your mark now thinks that their card id on the table, but you know it's still on the bottom of the deck. Once they pick a number use the glide again and deal out as many cards as they asked for, keeping their card on the bottom the whole time. Show it to them, and voila! Free beer!