http://www.catinfo.org/litterbox.htm

That site has a TON of info on cat litterbox problems and things to do at home to help (assuming it's a behavioral problem, not medical).

I tried the things on that site and I went from 7-8 accidents a week to a month accident free until the other night. I now have a HUGE rubbermaid tub with Cat Attract litter in it and the cats LOVE it. They practically aren't using any of the other 6 litterboxes at all, and best of all, only one accident in a month.