There's a quick and easy way to get the open jaw pose, that looks great, and reqires minimal tweaking. Group all the cp's in the face that would be affected by the jaw opening. Make sure you grab the lower teeth as well! Here's what I selected. (Might want to name a group for these points too).
While they're selected, hit R to bring up the rotation manipulator. Drag it's pivot point so that it lies just in front of the ear. Now if you rotate the points, you'll get an eerily realistic display of your characters jaw opening. Rotate the points until it's a little bit beyond what you think looks normal. That way you can go to a really extreme pose if your animation needs it, but otherwise you can just go to 90% of this pose, and it'll look ok.
You can see in that picture one of the core strengths of hash patches. The splines will seem to stretch and contract like real muscles, with very little effort on your part. Trying the above trick in a polygon based system would be almost impossible. I'm going to find it very difficult to do character work in any other package after this... =)
Deselect your points, and now scrub your frame slider between 0 to 100. Your character will smoothly go from his default pose, to the jaw open pose.
Now we'll define what happens at the negative extreme of the pose. Move the frame slider to -100. In this case, we want the 'hmmf' pose. From here on in, it's basically a matter of shunting cp's around until you get what you want. Make sure you have lots of reference material around. A mirror is handy. Just keep mugging at yourself, tweak a point, orbit the view, tweak again, orbit, tweak, orbit, rinse, repeat. A very handy book is The Artists Complete Guide to Facial Expressions, it clearly explains what muscles are pulling on what from where, how flesh and bone interact, and other handy little tips you won't find in a 3d cg book. Go buy it. Now!
Some quick tips:
Anyway, here's the 'hmmf' pose for my character:
