Steve Doherty blogged recently on releasing one's passion through painting, and his sixth point really rang true for me. Paint the same subject again, he wrote, and I've found that to be very helpful in terms of pencil sketch drawing as well. Steve's point was that the first painting of a given scene allows the artist to work out the mechanics of the composition—the drawing, the color mixtures, et cetera—freeing the artist to more expressively depict the scene in the second painting.
This is true for drawings as well, although the color aspect isn't as relevant. (Zeroing in on the right values is perhaps the equivalent.) But more important, for me at least, is the higher level of understanding of the objects in the scene that I get when I draw it a second time. If it is a model's pose, I've already figured out the basic proportions and discovered the tricky areas, especially any foreshortening. If it is a landscape scene, I am more familiar with what areas need to be greatly simplified to best showcase my focal point. If it is a scene with a lot of buildings (happens frequently, as I often draw and paint here in Gotham), then I have worked out the exacting perspective problems in the first drawing, allowing me to focus on other issues in the second one.
Even when the drawings are just two-minute sketches in a Moleskine notebook done before I tackle an underdrawing, I learn so much more from my second drawing. Here's an example:
Here is my first preliminary, two-minute drawing. I knew pretty quickly that it wasn't good enough to use as reference. I went ahead and drew it out to see if I could uncover other areas that would trip me up.

My second drawing fixed some proportion problems, more accurately presented the values of the paper birch that grows in the middle of the scene, and firmly planted that tree in the ground. I realized while doing this that the paper birch was one of the most important elements in the scene. Its characteristic white bark and the way its foliage creates the sky shape in the upper left were things I would have to deal with. They are both rough sketches, but the second one, which I actually executed much faster, portrays the scene much better.

Anyone else care to share two drawings done in quick sequence, showing the new observations and ideas that came with the second attempt?