Taking stimulant laxatives* isn't a good idea - even if they are natural.
The best prevention is a diet that is high in fiber, fruits and vegetables, low in animal products, and a lifestyle that includes regular excercise.
Plain old fiber is the best "cleaner" of your colon available, it sweeps things along through your intestinal tract. . . . and while its found in many foods, metamucil (or the walmart brand) can be taken in addition to a diet that includes plenty of fruits and veggies to increase your fiber intake.
*herbs such as senna, cascara sagrada, turkish rhubarb, etc. . .