Here are two great threads on the subject over at CG-Talk:
https://www.cgtalk.com/showthread.php?s=&threadid=20832
https://www.cgtalk.com/showthread.php?s=&threadid=34237
I have yet to complete a facial rig of my own (too busy doing other stuff), but when I do, I'll be sure to share what I learn up here.