Any time a cat is not using the litterbox regularly the first step should be a thorough evaluation by your cat's veterinarian.
The most common behavioral reasons I've seen for cat litterbox problems are surface and location preferences combined with litterbox aversion. This means that your cat (for a variety of possible reasons) has come to prefer surfaces (textures) and locations other than the litterbox. This is often secondary to a litterbox aversion that again, can develop for a variety of reasons.
If this is the case, and as Dr. Karnia said we need much more information (a private behavior history taking session in my consulting business usually takes about an hour and obviously isn't possible in this forum). BUT - if surface and location preferences are involved, a good starting point is to look at the litterbox characteristics and compare them to the areas where your cat is soiling. How are they different? What beahvioral need or preference do the soiled locations meet that the litterbox and its location do not? You have to think like your cat here!
An inexpensive resource to start with is Cats Come Clean: what your cat wants you to know about litterboxes - available here -
http://helpingkitty.com/booklets-cats-come-clean.htm
Most litterbox problems can be resolved IF a systematic approach is taken based on the reason for the inconsistent use.
Cats can urinate and defecate outside the litter box for many different reasons. It can be a medical problem, due to stress, or a problem with the litter box itself. Usually medical problems needs to be ruled out first with an exam and tests by your veterinarian.
If you can provide some additional information, we may be able to help more. How long has this been going on? Urinating or defecating out of the box or both? Normal stool? Normal urine? Any other symptoms? Any changes with the litter box or environment. Does your cat normally go outside? How many cats are in the house and how many litter boxes? What kind of litter do you use and how often do you clean it?
In general, cats like soft clumping litter, cleaned regularly (scooped daily and changed weekly), in a quiet space. Clean the spots where she went potty well with an odor neutralizer and try to block access to the spot. Remember, cats do not respond to punishment, so don't yell at her when you find it after the fact.
Write again with more information and we can try to be more specific.
Judy Karnia, DVM
Scottsdale Cat Clinic