Do you do Paul Harris' Las Vegas Leaper? You should, it's one of the best anytime, anywhere, any deck tricks you can do. I do the first phase as described in book 1 of the Art of Astonishment. Then I often do the second phase from Big Time Las Vegas Leaper, as described in book 3.
I have a fairly robust presentation for this effect, but today I'm just going to offer a small verbal tweak that can increase the reaction to this trick enormously.
First, for those who aren't familiar, the effect is that the spectator counts 10 cards in their own hands, the performer doesn't touch them after they've been counted. The performer counts 10 cards for himself and then transports three of his cards over to the spectator's. So when the magician counts his cards, he has 7, and when the spectator counts her cards, she has 13. Then they do it again. So the magician has 4 and the spectator has 16.
Here is the tweak. Once the spectator has counted their cards, you count off ten [supposedly] for yourself and hold up your pile. You say, "Ok, I have ten cards here. Name a number between one and ten."
Do you see? The numbers that will be said an overwhelming majority of the time are 3 or 7, and you are now set for a mindblowing effect because you already are holding 7 cards, and they already are holding 3 more than they think they are.
If they say 7, you say: Okay, I will take these ten cards and turn them into seven. [Squeeze the packet between your hands, concentrate for a beat, then cleanly count 7 cards to the table.] But those three cards didn't disappear, I sent them over to your pile.
If they say 3, you say: Okay, I will send you three of my cards.
If they say 3 or 7, I only do the first phase. It just seems so impossible. They chose any number they wanted while the cards were already isolated away. It's a perfect effect.
There are decent outs for all the other numbers as well.
If they chose 6 or 4, you are going to do the same thing, but you are just going to break it up into a first and second phase.
If they say 4, you say: Okay, I'm going to turn this packet of ten cards into four, one by one. [Make the cards vanish one at a time. False counting your pile as nine, then eight, then legitimately as seven. Pause.] The cards aren't really "disappearing," you know. Do you know where they're going? They're going to your pile. How many have I done so far? Three? Okay, let's see if it's working, so at this point you should have 13 cards. Count your cards. [Do the move to set up for the second phase.] Now, you said you wanted me to have four cards, right? So I have three more to go. [False count your four as seven.] We'll do these last three at the same time. [Do a little pop of the cards or whatever and immediately show you only have four. They now have 16.]
If they say 6, you say: Okay, I'm going to make six cards vanish. [Then repeat the same action as in the paragraph above.]
If they say 5, you say: Okay, I'm going to make five cards vanish. [Then you do the same actions as for four or six, but for the second phase you only do two more cards instead of three. Easy.]
If they say 2, 8, or 9 you're going to do something completely different but still logical.
If they say 2, 8, or 9, you say: Okay, let me see if I have one of those. [Then you spread through your cards to find a card with that value. For example, if they say "two" you then look to see if you have a two in your ten cards. You'll either have a card of that value in your packet, in which case you can remove it, or you don't, in which case let the spectator choose any of the cards you do have instead. So with a 2, 8, or 9 you're acting as if you weren't asking for a "number of cards" but rather a value of cards. Once you have a card of that value or the spectator picks any other card, you tell them to remember that card. You then position that card so that after you do first phase, it will be the card on the top of your spectators pile that you pick up and say, "And look, the two of spades [for example] is still warm." I'm being intentionally vague here, but if you have Paul's work you know what I'm talking about. That line comes straight from his routine. So if they name one of these values you act like you're identifying one particular target card to keep track of.]
If they say 1 or 10 [and they never do] you say: Okay... actually, no... pick a number somewhere between one and ten. That will make it more interesting.
The truth is, 70+ percent of the time they go for a 3 or 7 and this effect not only seems like a completely pure miracle, it becomes almost a sleightless trick. I don't do the one at a time vanish if they name 3 or 7. It's just one moment. "You said seven? Okay, these ten cards are now seven." or "You said three? Okay, I will send you three of my cards." Poof. You're done. Keep it clean. With 4, 5, or 6, I do the first phase as individual vanishes because you're splitting up "their" number into two phases anyway. So it makes sense to do individual cards up to three, check to see if it's working, then send the rest along all at once.
This post is probably confusing if you don't do the trick, and maybe even if you do. I really recommend learning the effect if you don't already do it. It's always been very strong for me, and this adjustment takes it to the next level. The only issue with Paul's original trick is that it didn't make a whole lot of sense to take ten cards in order to make three vanish. And perhaps the broad strokes of the method were too much of a straight line for the spectator, i.e. "I must have had more cards in my hand than I thought, and he must have known how many more." With the way this tweak plays out most of the time, your spectator will really feel like they chose how many cards were to travel. And when they feel that way there is genuinely no way to backtrack the method.