The mouth is one of the most expressive areas of the face. We first look to the eyes to convey attitude in the face but the mouth should complement the eyes and work with the rest of the pose. If you just look at the nightly news, no two people really speak the same. Mouth shapes should be unique to the character you are working on. You are striving for many different things in the mouth
Squash and Stretch: Its important to have the shapes squash and stretch in a natural way. I’m not talking about Bluth type squash and stretch. In cg, that level does not work as well. You want to feel it, not see it. You also need to know how to incorporate different parts of the face to make it look fleshy.
Arcs: Making sure the proper arcs are in place is so important. Pay close attention to the corners of the mouth when polishing dialogue. You really want those corners to move in and out in a natural way. Chattery mouth shapres are not fun to look at.
Design : Obviously designing good shapes is hard. You are looking for asymmetry, simple to complex shapes, straights to curves… You don’t want the mouth looking like a football. Yes, some mouth shapes may look that way, but the key is to have read-ability and appeal. Look at real life… Man people have very interesting shapes in the mouth. It adds so much character.
Simplicity: One of the most common student mistkes is to put too many mouth shapes into dialogue. If you just look at the way your mouth articulates shapes, you will see alot of shapes are combined.
Teeth: Know when to favor upper or lower teeth. Obviously they play a huge part in the design and acting of the character. Tom Wilkonson talks more with is bottom teeth showing, someone like Joe Pesci talks with his top teeth and Tom Crusie seems to have both teeth talking.
Again, all this comes from observation and just looking at great examples. Hope this helps a little.