Mr. John Collier notes from the book "The Art of Portrait Painting" about John Singer Sargent

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.