Just a quick suggestion to maybe help yourself get rendering.. If your drawing digital try to actually paint instead of using line. Make a silhouette blob sort of what you're trying to paint and then add in some lighter or darker values. If you're sketching on paper then try to get a good start on your drawing with line, keep the forms really basic then decide on your light source before you get to far along in the drawing.. drop in some values for shadows. Do tons of quick sketches where you get a good idea of what you're drawing with line and then drop in shadows really simply as fast as you can, go back and forth with line and tone as you're drawing. Or you could also draw with just values and then pull out some line for harder edges near the end of the process. Just mess around with it I guess, I'm sure you'll stumble on some way of working that you like and keep pushing on that.
You know ... I've been having the same question for years. I think it biols down the the grayscale, and how you apply it to lighting. How you apply it, I think it really is your own style, but then agian I dont know.
I draw much like you.
"As you may have noticed, Im not all here my self..."