Learning to draw means you have to unlearn how you used to draw. We learn to draw in stages, with different strategies at each stage...
Everyone learns to draw, more or less, by going through the same sequential stages - scribbling, drawing shapes, translating shapes into symbols - a circle with two dots becomes a face, add a crooked line and suddenly it's smiling at you. Usually, there comes a time when the aspiring artist realizes the limitations of this in capturing reality - everything is flat, nothing appears as it does in space - rounded forms, with complex surfaces that change as your viewpoint does.
With this drawing of a helmet, I accidentally stumbled on the secret - staring intently at the form of the visor, I suddenly saw it as a flat image of interlocking, jigsaw-like shapes, the contours of which created the illusion of three dimensions. It was so exciting! But I wasn't quite sure how I had accomplished this - it arose out of a period of almost trance-like focus, of allowing myself to focus only on the contours, translating them into lines on paper. Without the intention of doing so, I was able to go beyond the stereotypical, flat images of symbolic drawing, and to achieve the illusion of three dimensional space on a flat piece of paper.
Unfortunately, I found this to be an ability which was alluded me more often than not. The harder I tried, the less successful I become. But once in a while, it would re-emerge, when I was relaxed and focused. Not being able to reliably access this skill would become increasingly frustrating for me. I only really began to have control over this when I read "Drawing on the Right Side of the Brain," Betty Edwards' revolutionary guide to drawing. She explains the stages of drawing that we all go through, and methodically teaches this skill through examples and exercises.