I've already made these but they look great and are really useful. On the reverse of each card there is a square with the number of the card in large lettering. Of course each number appears four times, facing each direction, so it's not possible to tell which way it's orientated from behind. I've also colour coded each with a unique radial gradient (some nicely garish combinations). This means the students can remember their card number better and it also makes it a lot quicker to distribute cards. That and it makes the set of cards look really wonderfully colourful.
I made mine using Pages on the mac, I I imported the plicker PDF page and put the squares over the top to I knew it would print exactly on the reverse.
I also though about putting unique symbols (maybe animals...) on each card, again in all four directions, to make them even more memorable.
Lisa Sheppard commented
This is a great idea. Would you mind sharing your plicker card backs with me? Lisa