Hmm! I was just reading about this the other day, Ill type out what the article says:
You should have at least as many litter boxes as you have cats. That way none of them will ever be prevented from using the litter box because its already occupied.
If your cat begins to 'eliminate' in areas other than the litterbox, your first call should always be your vet, many medical conditions can cause a change in a cats litter box habits. If your vet determines your cat is healthy the cause my be behavioural. Most litterbox behavior problems can be resolved by using behavior modification techniques. Punishment is not the answer.
Some common reason why cats dont use the litter box are: an aversion to the box, a preference for a particular surface not provided by the box, a preference for a particular location where the box is not, or a combination of all three.
your cat may have decided that the litter box is an unpleasant place to 'eliminate' if
the box is not clean enough for her
she has experienced painful urination in the box due to a medical problem
she has been startled by a noise while using the box
she has been ambushed while in the box either by another cat, child, dog or you.
What you can do!
Keep the litter box extremely clean
Add a new box in a different location than the old one and use a different type of litter in the new box.
make sure the litter box isn't near an appliance that makes noise
If she consistently eliminates on a particular texture. For example, soft textured surfaces, such as carpet, bedding or clothing, try using a high quality, scoopable litter and put a soft rug under the litter box.
whew! Hope that helped a little!