Here is the simple answer, assuming you can get it: "Fiber One" breakfast cereal. It's better tasting and more effective than anything else I have tried. I'm not sure if it's Kellogg's or Post (I don't have a box nearby at the moment).

I have no horse in this race. I am not a cereal salesman. I'm just a very satisfied customer. I don't own stock in Fiber One. My only relationship to it is to go to the grocery store, purchase it (at full bust-out retail price),take it home, eat it and, well...you know how it ends.
Good Luck!