Via Smashing Magazine, I came across this post on: Fifty Clever Tutorials and Techniques on Traditional Drawing written by Aquil Akhter. (The link is older but I tested a few links and they did work).
The introduction to the post states: "This article contains a mixture of traditional drawing tutorials, drawing techniques and some methods for transforming and preparing your creations for screen design."
"Some are intermediate level and some are advanced tutorials that include general theory, useful tips, comic inspired art, pencil drawing, color processing, character sketching, doodles, shapes, proportional, perspective and more)."
You may want to check it out and get some sketching time in this weekend.