What's the best way to model a mouth for animation?

I’ve got my model all made, except for the mouth. I’ve been putting it off because I wasn’t sure how to go about it.
So, I need some help. How do I start it? I’m doing retopo right now, and I’m at the head. Any tips?

The character, by the way, will be talking. But I don’t necessarily need the tongue to move.

EDIT: I did look through Google first, but I couldn’t find anything solid.

Check out “Porl”
http://www.blendswap.com/3D-models/characters/porl/

A very simplistic, yet good to work with rig with face-shapekeys and a fully developed oral cavity with tongue, gums’n’teeth.

http://processdiary.com/2009/07/31/work-in-progress-july-2009/

http://processdiary.com/2009/08/27/work-in-progress-august-2009/