Generally the receiver is on his or her back with us. We do mix it up though. Sometimes I stand. When she receives, she will sometimes get on her knees with her head laying down or very rarely sit on my face. The variety is fun, but the best for both of us is probably laying on your back.
I think its fun when she lays down on her back on the edge of the bed or couch (or whatever) and then I stand just over her head and she gives me a sort of upside down bj. Plus I like the idea of being able to slide in and out of her mouth at my control and I get to play with her breasts (and get a spectacular view) while she works on me