Have you tried isometric bowel contractions? Just clench the entire length of your intestines hard and hold for as long as possible. Then work on peristaltic contractions, run them up and down the entire length of the bowels. This takes an incredible amount of body awareness and control, but there are monks in China that can do this, as well as regulate their body temperature, and slow down their heart beat. A guy on another forum told me that it only takes a couple of years of practice to get this down pretty well, but I haven't gotten around to trying it.