50 Sketching Resources for User Experience Designers Lists articles about Sketching, sketching tools, sketching templates, presentations about sketching, videos about sketching, books about sketching, sketching examples
Cardboard Modeling as a tool for (interaction) design. Includes detailed steps on how to create cardboard models. By Joep Frens, from TU/e.
How to Draw Cartoons contains a progressive set of pages illustrating basic cartoon drawing
How to Draw It is a site that contains various drawing lessons.
Sketches as Explanations by Commoncraft, uses sketches in videos to explain products, concepts and services. A good sampling that should inspire you to do the same.