This is based on a post I wrote over at ConceptArt.org a few months ago.
Realistic drawing in the Western tradition is dependent on the idea of a fixed viewpoint and the relation of all forms in the picture plane to that viewpoint. This is really what perspective is. Imagine a box facing you at eye level- you will only see its front plane. Move it straight to one side and you will see its side plane and its front plane. Move it straight down from there and you will see the top, side, and front planes. But then, if you keep it in that same location but rotate in certain ways, you might only see its front plane again.
This, combined with the idea of diminishing, is the heart of perspective. The exact implementation of this is all the vanishing points and horizon lines and eye levels and all that. While those things are very important to know, they're really secondary to this idea of the fixed viewpoint*. When you're drawing the figure, it's too complicated to figure out most of the vanishing points- you have to understand the first part well enough to estimate the effects of perspective on a given form. Figure drawing pretty much boils down to a complicated puzzle involving the cube example from above for each form of the body- but each form is much more complicated than a box. These forms change shape based on muscle flexion and forces from the environment, and are oriented in complicated ways dictated by the joints of the skeletal structure. Just take the most basic pose: a person of your height standing straight up facing you, with their eyes at your eye level. You will see the tops of their feet and shoulders, but not the top of their head.
Along with studying buildings and the other usual suspects in perspective, practice the simple rounded forms (cylinder, cone, sphere) bounded by a box from every possible angle, then move on to more complicated forms like vases. Really understand how each form fits within the box, and what happens to the ellipses in all possible cases. Then move on to more complex assymetrical forms with cross-sections that aren't circular. Most people blow this off thinking they understand it, but strangely can't draw a proper cylinder and unsurprisingly can't draw a figure except for a few static & memorized poses. It's easy to understand but hard to do properly.
The idea is to work out all the scenarios carefully ahead of time, so that when the model is in front of you don't need to spend time figuring it out then and there. What is a calf but a strangely shaped vase? And if you can't draw a simple vase tipped toward you in perspective without a lot of headache, how are you going to get the job done with the model in front of you (and probably with much less time)?
Spend the time up front and get these perspective scenarios into your vocabulary, so you're able to use them "conversationally" while drawing. It's important to be able to think this way, but not be mechanical about it.
* I recently found Bruce MacEvoy's excellent (but technical) articles on perspective over at handprint.com, and was pleased to see that he explicitly breaks out the Stationary Viewer as the first step in creating a perspective view.