Flat on the stomach, head also (no pillow) works well for me. I am able to rock my hips and raise my pelvis during the enema to help distribute the solution. Palms flat against the abdomen also aid in relieving cramping.
I find that if I am on my back, like during a coffee retention enema, then having my abdomen massaged by someone else helps greatly. If I try to do it myself it isn't as effective because I am also using some of my stomach muscles while moving my arms.