The best advice I can give is to start slow and gradually build up the amount of psylllium you can stand. Try using just half the amount called for until you get used to it. Do not take too much and be sure and drink plenty of water. If you don't get enough fluids you will probably have a difficult time with psyllium.
Personally, I like to do The Master Cleanse as a separate program from a bowel cleanse. Why don't you try a few days of Master-Cleanse alone, then gradually work in the shakes.