Glycerine-based soap seems to work best for me; I fill a 2+ quart bag, and take on water just until the first urge to evacuate begins, I then immediately visit the toilet to expel the older waste at the end of the colon, rather than push it back up and in.
I then re-fill the bag to full capacity, hang it fairly low (perhaps a foot or so above the level of injection) and try to take the full bag while lying on my back, If I experience an urge to expell, taking deep breaths or massaging the abdomen will usually dispel it.
Assuming the full bag can now be taken on, I'll then close the shut-off valve and continue abdominal massage, with the rectal tube still in place, for a few minutes before expelling again. The glycerine seems to promote the purging of the entire colon, as i usually notice the passage of "softer" material probably just passed into the cecum. If I'm desirous of a "complete" clean-out, I might follow the glycerine enema with one of castile-based soapsuds.
Finally, I'll finish with a rinse of plain water or baking soda. I view this as particularly important if strong soap solutions have been used, since the lining of the colon has probably been exposed. The end result might be the passage of a small amount of "clear" mucous, and usually leaves a hollow sensation centered around my navel.
Have never tried milk and molasses, but it would make an interesting comparison if the opportunity arises.