Keeping your cat well-hydrated goes a long way toward preventing constipation. Circulating water fountains are a good way to get a reluctant cat to drink more water, and wet food can add more moisture to the bowels.
Regular veterinary exams — at least every six months for an elderly cat — can catch health problems before they become severe. Cats should receive a rectal and anal sac inspection at each physical exam.
Dr. Muns, Margaret. “Constipation in Cats.” Best Friends.
Dr. Mahaney, Patrick. Interview. http://www.patrickmahaney.com/