Darren, here's how I performed it. The instructions are kind of "short" and I didn't include much presentation.
"I want to show you my latest invention. It's the world's thinnest deck of cards. You know, there's so much waste in a deck of cards. Forests across the country have been chopped down just to produce decks of cards. On the other hand, I've managed to compress an entire deck of cards into this tiny space..." Here you pull out the deck and show how thin it is. "Amazing isn't it?"
Pull out the six cards, holding them face down and spreading them. "You'd never guess that I could get an entire deck of cards onto these six cards. But I did. Of course, I had to squeeze things together a little bit." Here, turn the cards face-up, which gets a laugh, as the pips are jumbled together.
"But seriously, this deck will do everything that a real deck of cards will do. In fact, I'll do a trick with it. Rather than asking you to pick a card...that might be confusing with these cards...I want you to simply think of one. Think of any card in the deck. Don't try to make it obvious or simple. Just concentrate on any of the fifty-two cards, and remember it."
Now, spreading the cards face-up, pull out the two cards that have only pips on them, putting them aside.
"Let's not think about the pips yet, the suits. Just think about the value of your card. I'm going to hold these cards up, one by one. Just look at the values on them." You've now got just the four cards with numbers. "I want you to pick your card, so if you see that has the value you're thinking of, take it and put it in your pocket. If it doesn't have the value of your card, just put it face-down on the table here." You point to a spot, indicating where they are to put the cards.
Turn your head away and, one-by-one, hand them the four cards. As they take them, reiterate the instructions. "Remember, if you see the value, take it and hide it. Otherwise, place it on the table face-down."
When they've finished, turn back. There's now at least one face-down card on the table, and probably more than one. Pick them up and spread them casually towards the spectator. "Now, I'm not going to look at any of the cards, but you're sure that these cards didn't have your value, right?" As you do this, add the values from the marks on the back.
Pick up the other two cards and mix them in with the remaining cards in your hands. Don't look at the face of any cards. "This time, I want you to only look at the suits. Diamonds, hearts, clubs or spades. If you see the suit you're thinking of, take the card and put it in your pocket. Otherwise, put it face-down on the table."
Repeat the process with the cards.
When you turn around, you already know the value of the card. I start right away divining the value of the card. "You're thinking of a high card, a number card, but a high number. Don't tell me if I'm right or wrong. Concentrate on the suit." Pick up the cards on the table and spread them again. "You're sure you got all of them with the right suit?" Check the marks and identify the suit.
"It's a black card. Well, that's a fifty-fifty guess. So I'll do better than that. It's a club. Don't tell me if I'm right or wrong, but you'll notice that I haven't made a single mistake yet. Yes, I can tell what card you're thinking of. It's the highest number in a deck, the ten of clubs!"
I've never had anyone find the marks. I'm sure it's possible, but the handling allows you to conceal this nicely. Be sure to emphasize that you're not looking at any cards. I shuffle them slightly in my hands as I turn away, to point out that they're not in any order. Part of it is that you're reading marks on the discarded cards, not the chosen cards, which are hidden in their pocket. Also, the marks are easy to spot and add with just a casual spread of the cards as you show them again.
Good luck with it.