Unfortunately, there is no simple answer to what seems like a very straightforward and simple question. Constipation in cats is not a disease in and of itself. Rather, it is a sign or symptom of another problem. Causes of constipation will vary depending on the age and overall health of your cat. Sometimes the problem is as simple as a food intolerance or a diet that is insufficient in fiber. Most often, however, constipation can be indicative of something else that is going on.
In older cats, a common cause is a difficulty in maintaining adequate levels of hydration. This can happen for a variety of reasons, but usually low hydration levels are due to problems with the kidneys-they can no longer properly concentrate the urine, and an overall loss of fluid occurs.
Another possible cause of constipation in cats is a disease called "megacolon." Megacolon is a muscular disease of the large intestine in which the intestine cannot contract properly, leading to a backup of feces and thus, constipation.
So you see, there is no simple remedy for constipation without finding out specifically why the cat is having trouble with its bowel movements. Your cat should be taken to a veterinarian for a complete physical. The doctor may recommend some tests based on what he or she finds. Reasonable tests include bloodwork to check liver and kidney functions, and perhaps X-rays to look for megacolon. In the meantime, a high fiber diet can be fed to help promote intestinal (colon) motility.