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Glycerin (start in the 7-10% range) makes any enema crampier, more effective, and if it isn't rinsed out it will make for return trips to the toilet. You can mix glycerin with soap OR Epsom salts for a more effective solution (but don't use both, soap reacts with Epsom salts. Use Docusate or baby shampoo if you want all three). One member does some straight glycerin. He does it at the beginning, but with a "Y" connector and a small container or bulb you can put it in in the middle. He says it is quite the experience.
Baking soda and vinegar is an experience. Start with the nice, soothing baking soda enema - a tablespoon per quart. Don't put too much in. Then finish up with a vinegar or lemon juice enema, start with 2 TBSP-1/4 cup per quart, as you can always go up.
Two bags, one normal soapsuds one "double strength" can be used to effect. Give some of the normal and intersperse with "shots" of the strong. Make sure it is real soap and no funny additives. Alternatives are baby shampoo (not as crampy) or docusate (Colace - use 750-2 grams per quart. It will kick anyone's ass!)
Bisacodyl in the enema can give a powerful and eventually irresistible effect. 20mg is a good start. You can either get Magic Bullet suppositories or the small pills. If you do the pills put them in a small container with a squirt of Bronner's for a day or two. After that they're all broken down and the bisacodyl is much more available to the bowel then it is if you merely crush them. Mix it in an enema (I use a soapsuds enema, really effective) or do it as a "slug" with the enema coming very quickly after the bisacodyl and either way the urge and cramps will continue to build. You can also do two Dulcolax type suppositories about 10 minutes before the enema as another option.
M&M (hard to clean up) or Mayo (1/2 c bicarbonate, 1 C brown or white sugar, 1/2 or 1 quart water) can be very powerful as well.