@Joe: I just need more practice, that's all. Drawing only from life after the methods presented in Betty Edwards' book however is not sufficient practice because that only trains, as you know, ones' own perception and ability to see whatever is there but only as flat 2D shapes, as the contours which are formed as a projection of the real world volumes onto a flat surface. The result appears 3D if the perception and translation to pencil strokes was accurate but it does not mean the artist understands the volumes or that he/she could render them accurately from a different perspective without the help of a different reference photo.
I am aware of the fact that it isn't necessary to have any constructional knowledge to arrive at good life drawing skills (given many hours of practice of course) if you just draw what you see and keep making sure you really see it right. But that is not the main focus here.
The main focus/goal is on arriving at an understanding of that which is there as 3D volumes of width, height and depth and to be able to quickly render those from any angle from imagination, even angles which are completely different from the referenced real world object and angles from which an object hasn't even been seen before by extrapolating from what is already known and by imagining how it might appear from that unknown angle.
Now, if, and I stress IF I already were an expert at that, at spotting and correctly placing/feeling the volumes as they extend into the space in all directions, there would not be much difference in either drawing what is seen as a flat 2D image or drawing what is seen as 3D volumes first and then rendering the contours, lights, shadows and halftones based on those volumes.
The latter would have the advantage (over a mere 2D copy) of enabling the artist to render the referenced real life object(e.g. a photo) with a different light source or from an entirely different angle.
@wishie: Posemaniacs is great (worked from that in 2009 a bit (as seen on some older page of this thread)) and I should also get back to working from that again. I'll also check out the other site you linked to which seems interesting as well (and it features real naked people too!).
Working at bigger size is also a good idea (although in theory it doesn't make a difference since an accurately constructed or copied image should feel right at any size).
Now here are the results of todays practice.
First is a drawn comment on "your images are rife with symbolism":

Second, some 30sec posemaniacs studies:

Third some quick doodles trying to imagine how that same old pose would look from different angles (see... if I already had the skill I'm training for, these would look right instead of awkward

):

And fourth, oldschool flat drawing practice from reference photo without any construction practice involved (failed to capture her likeness again, I blame a lack of practice ^^):
process video:
http://www.youtube.com/watch?v=9u0U2hq13twresult:

And finally another deskshot (with a new solution for fixing the camera in a place where it doesn't get in the way):
