With my ex-wife giving, the go-to position for a bag enema was always supine, legs up, knees bent. A pillow under my rump shifted things enough toward the horizontal to give gravity assist to my legs ; that made the position relatively comfortable and easy to maintain without supporting my feet against a fixed surface, like a wall or headboard. I suppose stirrups would have been even better, but we never went that far with dedicated equipment. Of course, that was 20-odd years ago, when I was a lot more flexible than I am today. I was also an avid year-round bicycle rider -- road bikes -- which kept me in a position that more or less imitates the one I'm discussing, albeit rotated about 180 degrees. Cycling of that sort probably conditioned the involved muscles in a beneficial way.
The on-back position was best for me because it made everything visible to me. I could her watch her fill the bag, lubricate the nozzle, scoop out a gob of Vaseline and make a beeline for my anus with it. I'd see, not only hear, the click of the clamp as she opened it. I'd watch the bagful of cloudy Ivory solution slowly collapse -- it was an amber latex open-top type, folded along the sides, so it would go flat when emptied. Added to all that visual stimulation was the fact that I could maintain eye contact with her, and watch her mouth and facial expressions as she engaged me in all kinds of provocative talk. One other important benefit of this position was that it gave her unobstructed access to the parts of my body that so deeply appreciated the fondling she'd do with her talented hands.
I'm aware of the arguments regarding the optimum position(s) for inflow and retention, but we were not at all interested getting the best clinical results. We were doing it, quite simply, for maximum erotic gratification.
As for a bulb enema, the on-back position always seemed a bit awkward for that, mainly because, to align the nozzle with the rectal tract, the syringe needs to be held bulb down/nozzle up. Clearance between her hand wrapped around the 8-ounce bulb and the mattress was not quite adequate as I was positioned. She did eventually find a 6-ounce bulb that was easier for her to use all around -- but the best position for the bulb, as we did it, was prone, again with a pillow under my pelvis. The visual part of the proceeding was pretty well gone, but the audio was so much better than the silence of bag! She was a very (and deliciously!) noisy manipulator of the bulb, playing it like an instrument with each refill. Its sounds are so distinctive and evocative: as it's squeezed to empty it of residual solution, back into the bowl; then squeezed again with the nozzle immersed, to expel air. With the bowlful of solution kept on a nightstand close to my ear, even the drawing in of fluid made a slight gurgling sound that painted a vivid picture in my mind -- and, it put my nose that much closer to the source of that powerful aphrodisiac, a rich solution of Ivory in warm water!