Original Post: 2/28/2010
The mouth is a close second to the eyes as the most expressive feature of the face. Learning from masters as Andrew Loomis and Giovanni Civardi really helps. Here are a number of sketches from their instruction references.
Stan Prokopenko has a wonderful tutorial on drawing lips.
Here is my realistic attempt using his references.
Emphasizing the planes or curvatures of the lips will help with creating really 3-dimensionality. How light plays off the curves will bring more depth to them. You can "feel" the fullness of the form. The shadows under the bottom lip are as important as the lips themselves to create the illusion that the lip is not a flat surface.
One more item that I didn't understand or realize the importance of, the node. Stan's tutorial explains how the muscles of the face all connect to the corners of the mouth. This is called the node and he compares it to a bean. By emphasizing the wrapping of the muscles around the corners of the mouth can bring the smile to life and depth to the cheeks.