Basic Drawing Techniques : Line Drawing:
Directional strokes. To look convincing, these strokes should follow the edges of the plane or form they're describing, and should be varied in value or weight to suggest areas of more or less shadow. When using directional strokes, keep things simple by drawing only those lines that you feel are absolutely essential for capturing the feeling of the subject's structure.
Hatching and cross-hatching. Variants of directional lines, this pair of related techniques comes in handy for creating changing areas of value. The most basic of the two, hatching involves applying parallel strokes to create tonal areas. The closer together the strokes, the darker the resulting area. Like hatching, cross-hatching begins with a series of parallel strokes. But here, the relative darkness is created by overlaying a second set of lines at angles to the first. Once again, the closeness and density of the lines create areas of relative lightness and darkness.
Scribbling. This is another technique that's great for creating the shifting values that produce the illusion of form. By overlapping loose, freehand layers of scribbles, you can achieve a more three-dimensional look.
Blending. Begin with light strokes you don't want the lines pressed so hard into the paper that they can't be removed and use facial tissue, cotton balls or swabs, chamois cloth, stumps of varying sizes and your fingertips to blend. No matter how you do it, blending adds a final sculpted or modeled touch to your drawing.
By working to master the use of line, you'll build the critical ability to capture the essence of your subject in as few, or as many, strokes as you desire.
~~Tips from the Artist's Magazine