I like laying on my stomach and him entering me from behind. Its really deep that way
Melliegirl has my favorite - I enter her on her stomach and she can arch her back to get me in deeper. This way I can grind into her or be on my knees to thrust hard.
But variety is key - nothing beats the view of doggie, missionary is awesome for the face to face contact, her on top is the best for her to grind and cum, and spooning can be such a nice sensual way of anally penetrating her while holder her close!