Cats are meticulous creatures and usually are pretty darn good at cleaning themselves. However, if you find that your cat has gotten herself into a mess that is beyond her grooming means, say, a massive pile of mud, it is definitely time to take matters into your own hands.
Even if you're feeling lazy, don't ever use human shampoo to wash your cat's coat. Although people shampoo is not always toxic, it is formulated specifically for humans -- not cats. Cat skin doesn't have the same pH balance, and as a result, human shampoo can be very aggressive and drying on them. This could lead to flakiness and painful irritation for your precious pet -- not pleasant.
Be smart and consult your cat's veterinarian before purchasing a shampoo. To keep your cat's coat healthy, beautiful and oh so squeaky clean, the vet may suggest a mild shampoo that has no fragrance, as scents can often be very overpowering. Always buy a shampoo that is made only for cat use. For example, dog shampoos are made especially for canines, and may not be effective or safe on all types of animals.
Remember that wee little kittens are not exactly the same as adult cats -- their skin is more delicate. If you plan on bathing a dirty baby cat, look for cat shampoos that are made exclusively for kitten use.
It's no secret that most cats absolute abhor water and bathing. In fact, your usually gentle cat may even hiss at you as you attempt to bathe her! Since cats can generally handle their own cleanliness routines just fine, keep actual baths at a minimum. If you have a short-haired cat, do not bathe her more than once every six weeks. If your cat has longer hair which gets dirty a lot easier -- think Persians or Himalayans -- you may need to bathe her a little more often.