Lips        

Lips frame the outflow of verbal communication, the mouth. Your mouth also is used to smile and make facial expressions showing your current mood. A more beautiful mouth can really help your entire presentation.

• Lips are a visible organ at the mouth of humans and many animals. Both lips are soft, protruding, movable, and serve primarily for food intake, as a tactile sensory organ, and in articulation of speech. (Wikipedia http://en.wikipedia.org/wiki/Lip)

To see where the mouth is positioned vs the other facial features, I found this page on the net made for art students. It can help one know proportions and such. Some of the later rules of making up the mouth will be based on what your current proportions are as well as other features. Facial Proportions. Another page on proportions with good triangle of nostriles, lip, eye relation.

Proportions has the mouth located 2/3rds up from the bottom 1/3rd of the mouth. It is normally as wide as the center of the eyes. From the side, they have the upper lip idealing sticking out further than the lower lip. The lower lip also isnt as wide as the upper.

As for the shape of the lips, I use this page from permanent makeup page I found to see the different shapes.

Lip shapes. Men normally have a very thin upper lip, women more fuller. Men's mouths are also normally wider then women's. Another showing major feature postions. Page on Anatomy of Beauty.

A good thickness of the bottom lip is 1/3rd the width of your eye.

With knowledge of width, shape, and thickness of  lips, you can use corrective makeup techniques to create more ideal lips you desire.

Basic corrective techniques

http://cosmeticmagazine.com/managecategory.asp?C=360

http://www.101lifestyle.com/beauty/makeup/makeup3.html

Important tips for great lips (dry, color,

http://www.hindustanlink.com/beautytex/lips.htm