It is very easy for a person to forget which Aces are on the table and which are still in your hand, especially if you're working for an audience that isn't particularly attentive (or under the influence of alcohol). No effect will be perceived if this happens, and the trick will fail to impress your audience.
Therefore, it is imperative that we find a way to emphasise the relative location of the Aces before the transposition takes place. This must be done subtly. Otherwise, people might wonder, quite rightly, why you’re pointing out the obvious and anticipate the outcome of the trick. Such a realisation would make the trick a lot less magical and make it easier to reverse engineer the method. So, how do we make the magic clear without spoiling the surprise ending?
The simplest way to do this is to introduce some kind of visual reminder; a black casino chip would work well. Give the chip to a member of your audience and instruct him to place it on top of the card he believes to be the Ace of Spades. The idea here is that the black chip acts as a subtle memory cue that the black Aces are on the table. This prop also makes the trick more compelling because the cash value of a black chip is one hundred dollars, a big bet by most people's standards!
You can also use verbal memory cues to help your audience remember which Aces are where. For example, if there is a chair nearby, try dealing the two “black Aces” (?) to the chair and get a member of your audience to sit on the cards. Tell them that “the black cards are under your bottom.” As you deliver this line, emphasise the alliteration (the repetition of the “b” sound in “black” and “bottom”). This repetitive sound helps clarify the relative location of the Aces. Using a chair in this way is a fun way to stage the trick as the situation has built-in humour and makes the transposition much more impossible. The only reservation I have with this idea is that we might be back in too-perfect territory.
Another solution is to use different cards that are easier to remember. Bill Duncan has an excellent idea along these lines in his book Tubthumping, which involves presenting Dr Daley's Last Trick as a demonstration of hand mucking. I think this is an excellent idea because not only does it add clarity, it also justifies the use of four cards. In addition, this significantly improves the internal logic of the trick. (This pseudo hand mucking presentation is explored in Chapter Eleven.)