We train our students using an optical method of drawing called sight size. It is a very good method for the type of accuracy required for a portrait, and is also great for atmosphere and “seeing the whole”. Although it is not particularly dynamic in itself, once you have trained your eye you will find that your ability to create quick, expressive sketches will have improved tremendously. I found a reprint of an old interview with an American artist Robert Douglas Hunter (the whole interview can be seen here)
He says: “it is a method of viewing the model and your painting simultaneously from a selected position so that both images appear the same size. The artist is afforded a much clearer comparison of the subject to the painting, which eliminates transposing the visual image to a different size on the canvas as be paints. This allows the painting to be life size or under life size, because the size is determined by the relative position of the model, the easel, and the place you stand when viewing the subject, which I refer to as the “viewing point.” If you want your painting to be life size, the canvas is placed next to the subject; if it is to be under life size, the easel is moved nearer to the viewing point. That distance determines how much under life size the painting is to be.