hi thomas...
Check out this thread as a lot of what I said to TaKam. is pertinent to what you're doing here...
http://www.wayofthepixel.net/pixelation/index.php?topic=1007.msg12066#msg12066Specifically to your work here, what you need to consider is proportionally the head is too big for the shoulders... also with reference to the head... either the face is too large for the space or the cranium needs to be larger.... generally speaking eyes are in the center of the head, currently they're a little high... measure the distance between the chin and eyes and that's the height (roughly) you need fro the brain space...
Sketch the pose first as a flowing line that 'suggest's the overall posture, then fill out the form using spheres, boxes, cylinders and so on... learn to see through the figures and understand mass and finally buy a book on anatomy. Personally speaking I always found the muscle groups that make up the shoulder, armpit, chest (as you have here) were difficult for me to understand... Likewise the muscle groups that join to the elbow from the perspective you've chosen can be quite difficult to understand... as far as the wrist and hand is concerned you need to understand how the skelatal form of the radius and ulna work and link together with the tarsels of the hand...hence the use of a mirror... use yourself as reference - admittedly you may not be as well built as the character your trying to portray but it's a start and one of the good things of working this way is that you'll begin to FEEL the lines you're sketching... (sounds a bit weird but its true) - bottom line with anatomy is that you need to see BENEATH the surface and understand how the complexity of form, mass, and the mechanics of movement. Once you begin to grasp this your work will improve dramatically.
Additionally I would recommend you may be better off trying to fill in the rest of the body too as this will help with your overall composition and gain a better understandinf of anatomical form.
Keep it up and Good luck.