During the weekend I have made a series of experiments with my wife using ZENER cards (cross, square, circle, waves, star).
In this experiment I would shuffle the cards and then project one on the top to my wife. My wife had a 100% success rate receiving and calling out the correct card.
The interesting thing I have noticed in case of ZENER cards is the ease to keep each individual card in the plastic visualization. Hard to say whether this is caused by the superb design or the fact that these cards are used strictly for this type of use and got some egregores built for them.
Anyway the use is really simple. For the selected card I have visualized a plastic image I held at the center of my wife's third chakra (sometimes also slightly above on the forehead). As I already said, it was fairly easy to keep the visualization strong and effects were amazing. I really recommend experimentation with ZENER cards.