Thanks JFrancis for those links.
Here's my idea: The most important thing to understand is form. Understanding form and representing form on paper are different things. Representing form is a language, like any other, without that language the thought would still exist but could not be expressed. The thought is more important than the medium, you could describe form with words if you wanted.
"As we go on, you are to strive more and more to realize the manner in which the form fills the space. There wouldn't be any real movement without space for the body to move in, and there wouldn't be any form without space for the body to occupy."-Niccolaides, "The Natural Way to Draw", page 93 in my book. I've seen this idea stated different ways by different authors, it most be true. We must start somewhere so strive to understand space first, but of course it, form and movement are interrelated and will reinforce each other. Space can be measured by points and lines in space, on paper in perspective, remember points and lines have no mass or weight and therefore don't really exist but are abstract concepts used to measure space. Jack Kramer gives this a whole chapter in, "Human anatomy & figure drawing; the integration of structure and form".
Contour drawing, mass drawing, and modelled drawing in Niccolaides book are useful for understanding form. We learn from experience, these exercises are a way of having an experience when we draw. These experiences are then translated through the language of drawing:perspective, value etc.
Other thoughts:
Proportion is derived from the skeleton, so it's not possible to have a true grip on proportion without understanding the skeleton. The skeleton is also important for knowing the insertions of the muscles. Your studies must make logical sense, for example if you don't know about the rhomboids and some other small muscles of the neck then the trapezius shouldn't make sense to you. If it doesn't make sense it won't be remembered.
My other advice is to draw moving things, form doesn't chamge when it moves, in fact when a thing moves it is actually more truely itself than if it were doing a pose, birds are good for this. You don't even need to draw to study form, although the drawing reinforces your thinking and helps you remember, like taking notes as we all learned in school.
Understanding convex, concave, and straight lines is useful to, explained in R.G.Hattons "Figure Drawing". Having a way to measure/figure out interaction of masses is also useful, "Force: The Key to Capturing Life Through Drawing".
This is the first time I've tried to write any of this out, I hope some of it was useful. I hope we can all be killer artists!