For example, here is what he puts under drawing basics, which is only the beginning of his substantial list of items:
- Draw what you see, and if you can’t see, draw what you know
- Show soft corners, nearest observer
- Observe angles, arcs, verticals and horizontals
- Observe both sides of the form, draw into the form, not just the contours
- Use structure and symmetry
- Use overlap
- Use errors to improve the drawing, start lightly and don’t erase
- Get the gesture first
- Move from general to particular
- Increase the contrast. Make all areas in the light a little lighter than you see them, and all areas in the shadow a little darker than you see them. The object is to make all lighted areas hold together as one group, as should the shadow areas. Otherwise, the subject will not hold together; it will lose validity. – Fred Fixler
- Even in a sketch, have something implying the background
- You need to know how to make something unimportant as well as important [focus]
- Should not have line driven style and shape driven style in the same picture.
My advice is draw as much as you can, even if you're a sculptor, painter, or if you do installations. Drawing helps build not only skills but also ideas, too. I hope the link above, which has many more than what I listed here, helps you out.