http://www.fanciers.com/cat-faqs/medical-info.shtml

Diarrhea
If your cat has persistent diarrhea, take the cat to the vet if symptoms
have continued for more than 2 days. Bring a stool sample with you and have
the vet check for parasites and/or fever.
You can try changing (temporarily) the cat's diet to one or more of the
following (depending on the cat's preferences):
boiled rice
cottage cheese
bread
plain yogurt
boiled chicken
chicken broth
baby food (strained meat varieties)
The emphasis on the above being as bland as possible. No spices allowed as
they tend to aggravate the stomach. This procedure may be advisable to
reduce the possibility of dehydration from the diarrhea.
The vet may or may not prescribe medication. One-half teaspoon of kaopectate
(NOT peptobismol, it contains aspirin) usually works pretty well too. The
vet may recommend withholding food for 24-48 hours to give the GI tract a
rest before starting with some bland food.
Usually diarrhea lasts only a few days. If it lasts longer than that, as
long as the cat does not have a fever, it usually does not mean anything
serious, but you must protect the cat from dehydration by making it take in
plenty of liquids