Witwer had an original method of composition. He carried small scratch pads and short pencils in his pockets. If a comic idea occurred to him on the sidewalk, at a party, in conference, in a taxicab, in a speakeasy, or anywhere else, he would thrust his hand in his side coat pocket, make a note on the pad, tear off the sheet, and leave the pad in readiness for the next idea. Whenever he heard a very bright remark or a very dumb remark, Witwer’s right hand would dart into his coat pocket.
From long practice he could scribble legibly and inconspicuously.
After accumulating a hundred of two hundred of these notes, he would seat himself at his desk, cover the floor around him with the slips of paper, and start writing. When his invention lagged, he would lean over and pick up a slip of paper. If the paper failed to suggest anything useful at the moment, he would toss it back on the floor and pick up another. Sooner or later he would find a note which would inspire him. Once used, the slip would be crumpled and thrown into a wastebasket.
Quoted in the excellent blog.pmarca.com.
I follow the very same procedure with a notebook I carry everywhere. I often convert some of my notebook/journal entries into permanent notes for my slip box (zettelkasten).