I agree with Gnoll, do a few polished pieces to mix it up a bit.
I find that when I just do quick figure studies, I don't really think about what I'm drawing, just lines and shit, but when I work on a more polished piece I start to understand what I'm drawing. Obviously because I've been staring at if for hours.
Thats it mate keep it up !!! Its always cool to see line drawings , its difficult but rewarding to be good at it. For me masses are more comfortable than lines. Keep up the good work
If there is anything to suggest, is to study wireframe[greyscale] low polygon and high polygon 3d models, they contain all the necessary info