Well, the general thought to keep with you is that outlines simply convey boundaries of masses and shade defines the masses. Studies are usually more successful, I've found, when I disregard using lines and instead just shade the masses, any lines that result are borders from contrast and/or edge shadows, which is realistic. When drawing the human body, you have to be careful to not just draw the outline but also use light and shadow because otherwise you won't be learning the true form of the body, and the manner you perform your studies will not permit you to reach your goal in the fullest.
There's not much advice, I simply see shading as, when monochrome, marking areas of varying intensity... You just draw what you see, and that's really all there is to it. There's no rule on how much gradation or harsh shifts should be used, you simply draw what's there. Understanding it is another thing, and requires you to observe from a multitude of perspectives and go underneath what you see on the outside, understand how it influences the end product. The goal then becomes understanding the human enough to invent and modify with ease as to not to depend on reference out of necessity, otherwise one becomes nothing more than a faulty camera...