Good ole Metamucil is my fiber supplement of choice. Remember when chosing a fiber supplement that soluble fiber is good, and insoluble fiber is evil. Look for something that is 100% pure psyllium or methylcellulose. Both of these are soluble fibers. Personally, I've had better success with the psyllium (Metamucil) than the methylcellulose. There are other types of soluble fibers, but right now the names are slipping my mind.
Three tablespoons of the sugar sweetened Metamucil in a full 8 oz. glass of water, followed by another 8 oz glass of water really helps with my D and C cycles. When things get bad (either lots of D, or really C'd), I'll do a second dose of three tablespoons later in the day.
I've been taking at least three tablespoons of Metamucil every day for the last five years. It has done more to help my Chrohn's (both the C and D, and even cramps) more than anything except prednisone.
It has also lowered my cholesterol a bunch.