When I'm done with a painting and still have a palette full of paint, and a little time on the clock, I've started painting on top of postcards doing quick sketches. Sometimes I represent what was originally on the card, and sometimes I just use that as a jumping off point.
Last Spring I met an painter in SF at an open studio who uses a similar approach for her paintings. She mostly did industrial landscapes, so I asked if she painted outdoors or from photos. She said she took photos, had them blown up and transfered to big canvases and then she painted on top, to varying degrees. Some looked like they had very little paint on them at all!