I'm a strong believer in the knee-chest position for an enema.
Make sure you use knee-chest, not knee-head, or you'll get a sore neck!
The best way is to put a pillow under the knees and another under the chest, so that your head is on the mattress, in front of the pillow. If you're a female, you want to have the knee pillow just a little bit higher than the chest pillow, to allow for the breasts. Hands at a comfortable place away from the abdomen. Nothing restricting your lower abdomen. Best to leave your chest area unrestricted as well, so again, for you females, an everyday bra is optional, but nothing like a Wonderbra, or longline that restricts your breathing.