Lots of good advice in the previous posts, it's a question that I've wanted to ask someone for some time. I've used trial and error to find what works for me.
1. As previously stated, smaller amounts of liquid given in the knee-chest position, 5-10 minute retention
2. Salt water, or urine if you're into it are best for this, using my husband's urine was the solution that worked for me. No fleets, laxatives, or any "exotic" solutions that may cause more bowel stimulation than you want.
3. Patience is a virtue, after you think you're done expelling, wait at least 20 minutes, longer if possible. Use the time for other fun stuff.
4. Don't eat anything from the time you start cleaning out. If you must eat, eat a small snack that contains no natural helpers. I snack on soda crackers or drink an Ensure if I find myself hungry. A meal generally gets the bowels moving, and as a rule, they move south.
5. If all else fails, try going to a clinic for a colonic. You'll be able to have good anal fun for up to a week, plus you'll feel great. There is also a great "how to" in the library here on doing this at home. I haven't tried it yet, but I will be as soon as I have a helper. I don't feel like asking my husband to help with that one.
I always try to remember that it's natural. If one is going to be putting things up my butt, they shouldn't expect them to come out covered in roses. I've played with my husband's butt with toys and fingers, and have things come out less than clear, and I didn't think anything of it. I always keep a towel handy, the minute my husband pulls out, I throw it over his penis and rub. He gets mad at me for this, but if anything gross is to be discovered, I'd rather discover it. I don't want him to associate me with that.