Basic Drawing Made Amazingly Easy (Made Amazingly Easy Series)

Female lips, in the side view, require a delicate touch to appear
attractive. Fortunately, there are a few easy-to-follow rules of thumb
that produce classic beautiful lips. First, never draw thin lips—always
full. Second, avoid symmetry: In other words, make one lip thicker
than the other. It doesn’t matter which; in this example, I’ve chosen
the bottom. And third, keep the length short, but the width thick.