In cases of chronic constipation Dr Shelton used to advise an enema beforehand, although he was not an advocate of them generally (like the pioneers of Natural Hygiene) as they can be enervating and sometimes painful, but he made an exception in these cases.
Constipation can have many causes while eating, such as insufficient fiber or roughage within the diet, or lack of exercise, or as InCharge has already stated in reply: insufficient water or fluids within the diet. A diet that is predominantly of a high water content should also prevent constipation.