First, take a look at the top card, A good time to do this is when you shuffle the cards at the start of the trick. If the spectator insists on shuffling it for you, let them and then take it back and shuffle it again quickly, glimpsing at the top card while you're at it.
Then, hold the cards with your thumb on top and the rest of your fingers at the bottom. Now with your other thumb touching the edges of the cards close to the volunteer, explain that you are going to rifle through the cards and the spectator is to tell you to stop at any point in the deck that they feel like.
Now, slowly rifle through the cards till they tell you to stop.
When they stop you, lift up the portion of the deck that is above the stopping point. This may seem like an innocent move but it's not. Why? because while you are doing this you have slipped off the top card (which you have looked at already) with the thumb that was on the deck so that it is now on the bottom portion of the deck (make sure you don't look at the bottom part of the deck as you are doing this, it will make your participant watch it TOO closely).
Show the bottom card of the top portion(I know it's confusing but bear with me)to them and explain how this is NOT your card since you can see it right there and it would make a lousy card trick if you use it. Instead, offer them the bottom part of the deck and let them take the top card of that (which is the card that you memorized at the start of the trick! You did memorize it, didn't you?)
Have them put the card back where ever they want to. Now you can do ANYTHING you want to. Since you know their card, you can just pull it out whenever you want to. I personally like to force the card on them and then let THEM shuffle it as much as they want. After that I just glance through the cards and pull out theirs! This is especially effective when you're with someone who knows about the concept of key cards. You can also use this with many of the other tricks on this site. Just have FUN!!!