"Cultivate an ever-continuous power of observation. Wherever you are, be always ready to make slight notes of postures, groups and incidents.
"Store up in the mind without ceasing a continuous stream of observations from which to make selections later.
"Above all things get abroad, see the sunlight, and everything that is to be seen, the power of selection will follow.
Be continually making mental notes, make them again and again, test what you remember by sketches until you have got them fixed.
"Do not be backward at using every device and making every experiment that ingenuity can devise, in order to attain that sense of completeness which nature so beautifully provides, always bearing in mind the limitations of the materials in which you work."
Quoted from Sargent notes at Goodbrush.com
"Sargent in the Studio," an article about a 1999 exhibit at Harvard, with examples of Sargent's studies
More about the Boston Fine Art Museum murals at Brian Yoder's Good Art Gallery.