If retention for any period of time is what you are seeking, there a couple of things that I have done that works pretty well. First no one wants their bed covered in mess, So...I spread some sort of plastic under the top sheet ( a split open garbage bag works in a pinch). I arrange some sort of bolster combing towels and cushions that my patient lays over at her hips. She has not only her wrists and ankles restrained to the four corners, but I have also spread her knees and used under the bed restraints so that she not only has legs spread, but her knees held open. It takes a bit of time to get her semi comfortable, yet well restrained. The prep work is most important because once you start her fill, you don't want excuses for having to adjust and do fussy things that become a distraction.
From my own personal viewpoint, I get a real erotic jolt from having her in this open position....everything is on view. Next I lube her up and insert a Collins speculum in her vagina, this serves to help her into a relaxed state of mind (subspace in some cases). This speculum makes it a simple process to insert and inflate a catheter for when the urge to pee kicks in. I have a shut off petcock in line with her drain bag so that she is not drained until I sense her need. Again, visually, very erotic with her bag laying on the bed between her knees...I lean it up against her inner thigh so that she can feel the warmth of her urine as I drain her.
Next comes her double balloon rectal nozzle....the standby for retention of anything in her rectum. I hang the 1 gallon bag low on a hook so she gets a slow fill and use a twirling red ball connector that you can control the flow. I use plain warm water....no suppositories, no soap...just plain water. I want the fill to be slow and comfortable so that the beginning and the end of the enema blends together in one long session that is satisfying.
Once everything is ready, I put on some soft music and turn the lights off except for a lamp. I crawl into bed along side her and gently stroke and massage her buttocks and back. As the enema flows, I will slip a finger or two down through your vulva and maybe tease her clit.
This session works quite well and I have done it on several occasions, sometimes adding a gag and/or a blindfold.
Her orgasms may build and come at any time during the session , usually lasting about three hours....sometimes 4. I use a one gallon bag, but she usually takes only 2-3 quarts. There are a lot of emotions and exchanges that take place during this 3-4 hour, however, the enema is the basis for her entire session. Her total capitulation and loss of body function control creates a really sexy dynamic.