I've been slowly going through Mark Kennedy's blog, Temple of the Seven Golden Camels, and in March/April of 2007 he posted some really good stuff on sketchbooks. I would definitely recommend checking it out (honestly, the entire blog is full of awesome stuff). There are links to those specific posts down at the bottom.
One part that I thought was really cool from what he says is this little paragraph:
This guy was really cool looking. I don't think I ever quite captured him, but I sure tried!
I draw way too many floating heads. MORE FULL BODY GESTURES!!