I've started to think about how I want the face to work and the first thing I started looking into was the jaw opening. I figured I'd post a little workflow tip that I do when working with the mouth. The problem with the mouth is that we always model it closed. Then if you choose to open it with a joint you never quite get the right looking mouth open. Its usually because all the verts were collapsed in on themselves in the default pose, so when they untangle, they don't have the right thickness and volume....
So here's my workflow. I model my mouth closed as normal and dont worry about it too much. Then I add my jaw joint, bind it and do a basic weighting setup so I can get a nice jaw open (not the final, but enough to see the volumes)... Now I open the jaw, duplicate off that mesh go into mudbox or zbrush and model the volumes to look the way I want. Now, I apply this shape back onto the original as a blend target, making sure the jaw is closed. This way I can see how its affected the original pose and because the blend is live (still connected), I'm able to tweak it a little more and make sure that I'm also getting a good base shape.
Here are some pics to show the results, the shape on the right is the old mouth open before being fixed... you can see the shape is cleaner, has more volume and will create better shapes when added with other mouth shapes:
Here's a sticky post of my reel - look in the previous posts for the full posting: Rigging Reel