If you eat. Adequate fiber in your diet, ie fruits vegetables salads, whole grains, don't waste your money on metamucil or other fiber substitutes and don't forget to drink 80-100 oz of caffeine free/alcohol free fluid daily. Fluid is necessary for fiber to work properly. Even so this will not prevent 100% of the cases of diverticulitis but it will reduces your chances.
3 doses a day. Though i think that a high fiber diet &/or metamucil prevents diverticulitis, it is not conclusively proven. Diverticulitis is a peridiverticular infection while diverticulosis is the condition of having diverticulae. 3 doses of metamucil a day will prevent the formation of diverticulae.