There are lots of different ways to mark the location of a card. There are two basic methods: marking the card itself, or marking the space/cards surrounding it so you can easily find the card.
The spotting card is the easiest: all you have to do is quickly look to the bottom of the deck and spot a card. While the audience is busy with something else (or you're using your misdirection skills), you simply take a quick look at the bottom and remember that card.
There's two ways this can come in handy: you can later make sure the card the audience picked is right next to this card, so you can easily find it (you're using it as a location marker). Or, you somehow move the chosen card to the bottom of the deck, and quickly look so that you know exactly which card the spectator picked. Then, you can do anything you like to throw off your audience, because you already know the card.
The glide simply requires you to glide a card backwards/out of the deck, so that you can easily move/locate it. The common way is to use a glide on the bottom card of the deck, but it can be done on any card.
- Bottom Card: Use your pinky or some pressure from the palm to slide the bottom card backwards, toward you. It can be extended quite a bit, the audience can't see it if you do it properly.
- Other Cards: This is a bit harder. Subtly lift the cards on top of the one you need (with your left hand, deck lies in right hand), and use your thumb to slide the card backwards. The technique is often used to pretend to grab a certain card, but in fact you grab a different one. For example, if you glide the bottom card, you can now easily get the 2nd to bottom card of the deck, while the audience thinks it's the bottom one.
This move is in some way the same as the glide, but has some extras attached to it and usually serves a different purpose. What you do is, you make the card your 'victim' picked subtly stand out of the rest of the deck. To make this invisible to the audience, it's best to make it really stand out at the back of the deck (the part that's facing you), and keep the front the same as always. As you might have guessed, a glide is needed somewhere in that move. Here are some examples:
- Putting a card back into a fanned deck: You first let a spectator choose a random card from a fanned deck, (look at it), and then he must put it back somewhere in there. The trick is, to apply pressure with the thumb on the 'root' of the deck, so that the card can't be put back completely, a little bit sticks out. Then you can simply rotate the deck 180 degrees while squaring it (to cover this up). Then you have the card your spectator chose sticking out in the back.
- Putting a card back somewhere random in a deck: This is the other way cards are often returned to the deck. When the spectator puts his card back, you simply perform a mid-deck glide: while you put the top half of the deck back, use the thumb of that hand to slide the top card of the bottom half (the chosen card) back a bit. You can also push the card a bit diagonally, so that the chosen card rests on top of your pinky for further use. This however is tricky, because a diagonal card in the deck is easier to spot.
This is not exactly one of my favorites, but it is also an often executed method. What you do is, once you have the spectator's card isolated from the rest of the cards (using perhaps the methods above), you fold the right corner at the back of the card a bit. Then, you can just put the card back in the deck, and you can easily spot it. Be careful though, to grab the card at this corner when you display it to the audience. While you're pulling out the card, you can slide your fingers a bit to make the crimp undone.
Passes & Breaks
Besides all of these, there are a few location marking techniques that can used for other things as well, and are much quicker and more powerful. Those are discussed in the next chapter.