Barbara Tuchman, Practicing History:

…I take notes on four-by-six index cards, reminding myself about once an hour of a rule I read long ago in a research manual, “Never write on the back of anything.” Since copying is a chore and a bore, use of the cards, the smaller the better, forces one to extract the strictly relevant, to distill from the very beginning, to pass the material through the grinder of one’s own mind, so to speak. Eventually, as the cards fall into groups according to subject or person or chronological sequence, the pattern of my story will emerge. Besides, they are convenient, as they can be filed in a shoe box and carried around in a pocketbook. When ready to write I need only to take along a packet of them, representing a chapter, and I am equipped to work anywhere; whereas if one writes surrounded by a pile of books, one is tied to a single place, and furthermore likely to be too much influenced by other authors.