I read this in the Reader's Digest some time ago and it seemed to work:
Get some ceramic bowls (not plastic, not paper). Mix one cup gingerale, a TB of baking soda and four TB's of deli mustard. Put that mixture in each bowl. Then, place each bowl with the mixture in it in each corner of the house or apartment in question.
Here's the key: Every three hours, place two tablespoons of bubble bath in the bowl and stir for 60 seconds. I heard "Mr. Bubble" is best.
I guess the idea is the scent of the bubble bath mixed with gingerale and baking soda makes the cats hold their bladder longer than usual until they find the litter box.
Tell me how it goes.