I have cats that weren't so much marking as they were refusing to use the box. As TheRedQueen suggested, more boxes than cats at various locations in the house helped us; I also tried a variety of different kinds of litter-- from very sandy and fine to very rough clay, and scented or unscented, as well as with powder odor removing additives; I cleaned the box twice a day, and tried boxes with and without lids. I also ended up throwing away a lot of the fabric things that they were peeing on just to get a clean start. Eventually, we found that unscented, fine litter in an uncovered box, with one more box than cats (3 cats, 4 boxes) did the trick. I think one particular cat felt harassed and trapped in such a closed space. Over the last couple of years, we weaned them down to one box, but recently added another cat, and so have 3 boxes. As long as one of the 3 fits the profile, and they get cleaned out frequently (NOT twice a day anymore!), then all is well.
If an uncovered box is a problem, as it was for our kitties who peed over the side, TidyCat makes a box with a hinged front; we removed the hinged front and now it's only covered on the back half, but the front is wide open.
We now clean everything with Out! or Nature's Miracle. However, when our recent addition peed on our brand new feather comforter a few months ago
we found that Denver Mattress's Mattress Cleaner II was pretty great in combination with Out!, especially for something that we can't easily put in the laundry.
"In these bodies, we will live; in these bodies we will die.
Where you invest your love, you invest your life." --Marcus Mumford