I often find that if students have an image to look at when they draw, they tend to surprise themselves in what they can do.
1. I used the internet to find animal photos that had a variety of shadows. After downloading the image and turning it black and white (thanks Photoshop!) I cropped it right down the middle and printed one side on my laser printer.
2. The students are instructed to lightly draw the edges of the missing side first, then come back and shade the sections in with their pencil. Soft drawing pencils will help them get the darkest grays and blacks possible. Encourage lots of contrast. The more range of darkness in the drawing, the better it will look.
3. I've found that students initially think of shading in just "light" and "dark" terms. They are to keep looking for details of the many shades and textures in the photo.