So for story purposes, I need to have a hall of mirrors, where one character doesn't appear in on of the mirrors. This is my current attempt... http://mad-matt.com/CIG/index.php?page=119

But as you can obviously tell, that sucks. So does anyone have a tip or trick?

You probably need to make the blank mirror more obvious (like the one where they're short)... it's sort of like those games where you see 2 images and have to find the things that are different between them... so, make it something your eye is immediately drawn to.