You have strength of line, which is half the battle. Beyond that I would say to draw from life and try to find the contour. And Never Trace!!!
It took me years to to find the strength of some of your lines. Most art nowadays is weak in contour. with emphasis on detail.
1: If you let yourself go and use bold strokes your art would improve dramatically.
2: Remember line should always strengthen contour, not define it.
3: Charcoal is good for these studies.
4: Study light and shadow to grasp that rule. Look at Frazetta's work at net-cafe.hu to understand what I mean.
5: All art in the end is just the basic building blocks ie. "sphere, cube, etc." in pretty compositions.
6: Last but not least, is have fun.