Anne Lamott’s Bird by Bird: Some Instructions on Writing and Life (Anchor, 1995) mentions how how she uses index cards for more than just composition:
I have index cards and pens all over the house—by the bed, in the bathroom, in the kitchen, by the phones, and I have them in the glove compartment of my car. I carry one with me in my back pocket when I take my dog for a walk. In fact, I carry it folded lengthwise, if you need to know, so that, God forbid, I won’t look bulky. You may want to consider doing the same. I don’t even know you, but I bet you have enough on your mind without having to worry about whether or not you look bulky. So whenever I am leaving the house without my purse—in which there are actual notepads, let alone index cards—I fold an index card lengthwise in half, stick it in my back pocket along with a pen, and head out, knowing that if I have an idea, or see something lovely or strange or for any reason worth remembering, I will be able to jot down a couple of words to remind me of it. Sometimes, if I overhear or think of an exact line of dialogue or a transition, I write it down verbatim. I stick the card back in my pocket. I might be walking along the salt marsh, or out at Phoenix Lake, or in the express line at Safeway, and suddenly I hear something wonderful that makes me want to smile or snap my fingers—as if it has just come back to me—and I take out my index card and scribble it down.
Quote courtesy of mleddy.blogspot.com.