Scientists are trying to understand that very question. In a recent study scientists examined whether, as with osteoporosis, exercising or taking calcium could make a difference. But unlike osteoporosis, these things did not seem to make any difference. However, since dowager's hump (kyphosis) and osteoporosis are related, it would be a good idea to work on preventing osteoporosis by exercising, abstaining from cigarettes, and taking calcium. Until we know more about what causes kyphosis, this is the best available advice.