A person's mouth is probably the second most important focal point of the head, next to the eye. It often communicates emotion louder than the eyes.

 

One of the first things to make note of is that, in most cases, the upper lip is slightly darker than the bottom lip.

Since the lips curve, the outer edges of each lip is darker than the interior, except in some cases, most often with males, where the bottom lip has a very light edge.

However, don't shade the edge to the degree that it appears separate from the face. In all perspectives except the profile, the center of the top lip is often lighter than the rest because it projects out. In the three-quarter view and sometimes the front view, the shading changes at that center point. The bottom lip is always highlighted at some point, usually running parallel to the bottom of the lip.

One of the most common problems that people have when drawing mouths are the teeth. A frequent mistake is that people draw the lines of each teeth the way they see it, which creates an awkward look. The border' between each teeth should not be treated as lines, but shades. The front teeth are the biggest, and each will decrease in size as they are further into the mouth. Each tooth should be lightly shaded where they border the next tooth, and a little more into the edges so they don't appear completely flat. The back teeth will also be much darker than the front.