To answer my own question, two come to mind:
The first, as a newlywed, when my (then) wife gave me two enemas of two quarts each. I had become very constipated over about a two week period, and I was really stopped up. They worked wonders, but my ex, though a nurse, didn't want any part of "play."
The second, and even more effective, I administered to myself. I was away from home for a conference, and I was extremely stopped up. I went into a Long's Drugs, and found a Fleet Bagenema. I bought the enema (receiving a "flirting smile" from the cashier, which certainly made me think), and proceeded back to my room. I assembled the bag, added very warm water, and mixed in the soap. I got down on the floor (on a towel, I think), and let the water make its way into me. The Bagenema, though just 1500 cc's, was probably the most effective enema I have ever had. I emptied out over about the next 45 minutes, and when I was finished, I'm sure I had lost 10 pounds. What I don't know is how that hotel toilet managed to flush. I'm glad I wasn't rooming with anyone. The smells were enough to convince me that I could never say, "My shit don't stink," because I almost had to leave the room myself...
True story, of course. Happened in Palm Springs, CA.