I teach drawing from observation. I start off with talking about projection and drawing machines then show them how to create their own 'conceptual'mdrawing machine using nothing but the straight edge of a pencil. The trick is to identify points in the scene (intersections, corners, features etc) which corespond to each other on a vertical or horizontal axis. with a few of those done, the whole drawing drops into place.

Edit. Unless I am missing something, I see no mention of Van Goghs sighting grid. Crude, but effective.

http://www.vangoghreproductions.com/art-techniques/perspecti...