When drawing with a pencil, one only has three elements to work with: line, shape and value. Shading and hatching are ways to make value. Shading is the act of creating an accurate range of values with smooth transitions. Whereas, hatching is the means of creating values with a series of lines. With hatching, one's lines show and can produce a sense of energy or roughness. Shading is about a more accurate sense of how light might fall on an object. For an art lesson on hatching, click here. Shading is much like hatching in that the more lines or marks drawn in an area the darker it will appear. But the techniques of shading differ from hatching in two common ways:
- applying more or less pressure to the pencil makes values darker or lighter
- one can use a variety of pencils to produce a variety of values
Also, like hatching, shading works best when there is an organized manner in which marks are drawn on the paper. Stroking marks in one direction will help to create a uniform surface to your shading. If your strokes are very visible, it helps to shade across your first layer with 
a second layer of strokes in a different direction. Try not to stroke at a true 90 degree angle to your first layer. This can make your shading look like a screen or grid instead of the surface of the object you are drawing.
Edges are important with shading. To create a strong edge, the strokes should not go beyond the shape. In order to have a crisp clean edge, stroke away from the edge of the shape and then clean up by stroking carefully along the edge of the shape.
Sometimes ridges of darker value appear within your shading. Some areas of value are too big to shade in one quick row of hatch strokes. In these areas you need to work your way across the shape with a series of strokes. The dark edges I mentioned are caused by overlapping 
strokes as you work your way across an area. There are a few ways to avoid this. If the dark ridges are not highly noticeable, sometimes it is enough to add another layer of hatch strokes in a different direction. Another solution is to stagger your hatch strokes so that there is not an edge to overlap (see example). 
Then stagger your next series of hatch strokes into the previous series(example has darkened strokes to show technique. Second series of strokes should be same value as first series). The final solution for ridges is when you come to the end of your strokes, lighten up the pressure on the pencil to lighten the lines. This way, when you draw your next series of strokes the lines will be lighter where they overlap. With practice, the value of the overlapping ends will match the value in the middle of the stroke. I generally use a combination of all three of these techniques.
Shading Exercise 1: Practice shading by making shading boxes (like below). Boxes should be 1.5" x 3". Practice shading by making one box of solid value, one box that transitions from dark to light, one box that transitions from light to dark and back to light again and finally, a box with a line through it with both shapes going from dark to light.
Shading Exercise 2: Make a shading box that transitions from dark to light for each of the following pencils: 4H, 2H, HB, 2B, 4B. Then make one larger box (3" x 6") using all five of the pencils to transition the box from dark to light. Use the 4B for the darkest area and transition all the way to the 4H for the lightest areas.
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