Sean, in my opinion you and I seem to be at a similar level of understanding female anatomy. This is something I work on as well. I just posted a thread,
http://www.zbrushcentral.com/showthread.php?t=84683
And I wonder if you would have a look at some of my approaches.
I guess there are at least two ways of starting with ZSphere2 sketching. You can either use it to build up mass, disregarding the finer points of anatomy and structure in the beginning, leaving that for later sculpting steps, or you can try to build up the anatomy, at least the surface landmarks of the skeletons and major muscle groups, trying to build up as much structure as you can before moving on to the kinds of things you can only sculpt on the mesh. My preference is for doing the structure of bones and muscles while zsketching. My reasoning is that it is a great tool for nailing down the underlying anatomy, and that anatomy is of central importance.
To critique your work, nimajneb seems to have it right about your shoulders. Notice that the surface of your back between the arms also appears too narrow.
The bone of the upper arm, the humerus, has a ball that fits mostly into a part of the shoulder blade that juts out, and it is capped by the end of the collar bone. The collar bone and a spine of the shoulder blade meet at an angle here. <0> The deltoid can almost look like one piece with the pectoral mucles, but there is a gap at the inward curve of the collar bone, and from that gap it juts forward, along with the collar bone, while the spine of the shoulder blade moves out from a point about halfway between the shoulder joint and the spine. Depending on the position of the arm.
Your character’s forehead seems a bit narrow to me as well.
I would appreciate comments from you or nimajneb on my thread as well.