I would seriously advise you to work with graphite. It gives much more fluid and vintage look to your work. Doing sketches with pens to me just makes it feel unfinished and messy. I would only use pens when doing an outline of something (like what Proto said about H pencils). Besides you can always go back and undo something with pencils and not with ink.

That said, it's clear to see you have talent. I would suggest doing more drawings of the body. Give yourself a time limit of 30 seconds and start doing quick body proportion sketches and your understanding of anatomy and where everything goes will improve. All about training the eye.

Keep up the good work.