In the hospital where I worked we were sometimes called upon to make use of what was called the "Atomic Bomb" enema. It consisted of equal parts (adding up to 300ml) of glycerin, sodium phosphate and Castile soap. It nearly always lived up to its name, producing amazing results.

But if there is no particular hurry, just eating all the bran flakes cereal you can hold will take care of most problems. Strange but true: it works well for the "runs" as well.