Another way to draw from observation is by observing angles. In this video I describe the "linear-block-in", and how it can be useful for accurate drawing. The basic concept is to work from basic to complex: starting with a large "envelope" that encapsulates the entire form. As you work, the envelope is refined into smaller and smaller angles, which can eventually rounded out into curves. This is a very academic way of drawing, and can be a good practice for training your eye.