Drawing from the Old Masters Design workshop, what is probably the most important principle is that of clear sections of dark, medium, and light tonal values. On my first day of the workshop, I proudly presented a print of this image. Koo immediately said "No lights!" I explained that of course there were lights, "Just look at that highlight on the pear." After a couple more exchanges, I finally got it. A highlight on a turned form is not a light tonal mass. Each tonal value has to be some kind of object or mass of objects--not just pieces of an object.
This image is, I think, a good example of all three tonal values. And besides that, it also has some interesting black shapes as well as massed tonal values across the lily pads.
Here's a desaturated version of the pear image referenced above. See? No lights!