My favorite position, at least at the start, is lying on my left side on a leather couch (fully protected) with my belly just at the edge. This position give me access to my anus so I can easily fantasize that someone is giving me the enema. Imagining that a latex-gloved hand is spreading my cheeks to take my rectal temperature is compelling. Making believe I'm being told, after the thermometer is removed, that I need an immediate enema makes my penis quite hard. I've found that after taking a quart or more of fluid I can comfortably kneel (legs apart) on the couch with my chest against the couch arm and my belly hanging free. I took 4-qts that session. It was difficult and I haven't been able to duplicate that amount in a long while.
My enema position also depends on the nozzle I use. I have a greater
choice of positions if I use a retention nozzle like a double balloon
rectal catheter because I can keep both hands free. If I need to hold
the nozzle in place or if there is a liklihood of leakage I'm limited to
lying on my side (Sim's position) or on my back, typically with my legs
bent and partially raised. Now, if I'm being given an enema, my
favorite is the knee/chest so my belly hangs freely and my genitals are
exposed for the caregiver to stimulate (hopefully).