James Gurney has a lot of great technical info on painting on his blog!
A post of his with Notes from Sargent (also another good place for Sargent notes are the ones that Craig Mullins collected)
1. Painting is an interpretation of tone. Colour drawn with a brush.
2. Keep the planes free and simple, drawing a full brush down the whole contour of a cheek.
3. Always paint one thing into another and not side by side until they touch.
4. The thicker your paint—the more your color flows.
5. Simplify, omit all but the most essential elements—values, especially the values. You must clarify the values.
6. The secret of painting is in the half tone of each plane, in economizing the accents and in the handling of the lights.
7. You begin with the middle tones and work up from it . . . so that you deal last with your lightest lights and darkest darks, you avoid false accents.
8. Paint in all the half tones and the generalized passages quite thick.
9. It is impossible for a painter to try to repaint a head where the understructure was wrong.
PALETTE: Silver White, Naples Yellow, Yellow Ochre, Ochre dew (English Red), Red Ochre, Vermillion, Ivory or Coal Black, and Prussian Blue.
One of the keys of understanding Sargent is looking at the painting methods of his teacher Carolus Duran, who had a somewhat unusual method compared to other academic teachers. In a nutshell he tended to block in the tones in discreet mosaic-like patches at first (like a plane head, if you're familiar with those things) and then later in the game you blend the patches into each other. I believe this method gives the best tonal accuracy, which is what he's singing about in #5.
Regarding #7 about starting with the middle tones, I believe he means that right away you want to make your big value statement (or the 'effet' as they called it), but you should reserve your very darkest accents and lightest highlights for final, carefully considered touches.
and these interesting ones about horizon line /eye level showing where your camera is positioned (which is helpful for storyboarding as well) part 1 part 2 part 3 in practice