With the caveat that I'm largely self-taught, I always play standing up--I can't get the guitar at the right angle if I sit down. This might be another case of full-sized guitar versus not-so-big human (I had less trouble with my mother's slightly-smaller F-30). I've finally gotten past the point of needing to see my left hand and that frees up position a lot. I also tend to have the strap adjusted so the guitar is carried "waist-high", as mentioned by PoppaGator, to avoid bending my left wrist too much; I have small hands and short, crooked fingers anyway and having to keep my wrist bent means I have to work a lot harder to reach and "squeeze" the strings (this is on a steel-string guitar with a fairly narrow neck and slightly high action). The angle of the guitar neck away from my body, and the angle of the guitar neck away from the floor depend on how high up the neck I have to play.

Basically, how do I have to hold the guitar to get through a six-hour square dance without causing pain or numbness in my upper extremities?