When I draw something, it's almost never in one sitting. I always find that before I'm feeling like it's 'finished', I'm distracted, dragged screaming, or otherwise committed to other tasks at hand. This generally leaves me with a massive array of unfinished doodles, drawings and designs.
Since embarking upon my 'drawing a day' (which is a very loose definition) initiative, I've found myself doing bursts of thumbnails instead of full blown sketches in the hope of solidifying an idea in the back of my head. Aside from giving the false impression that I've accomplished something, this tends to mean I have several goes at the same idea, trying different angles, and thinking more about composition, not to mention giving me a great resource to fall back on when I feel like drawing, but can't find inspiration.
Even though I've now got several dozen little thumbnails sitting in my todo pile, I'm finding it's a pretty worthwhile practice. Next time I do some scanning, I think I'll share a few... if I find time.