I created a tutorial for this principle a long time ago on a site that shut down, so I thought i'd revisit it.
Shape stacking is a unique part of anime studio which makes me use it for illustration over Illustrator sometimes (that and point specific line weight) - its an essential principle that will save you the hassle of masking when you don't need to, and seperating things to too many layers when not necessary.
Once you do this, you can take the mouth into its own layer if you want - it does get a little complicated when you have that many points in one place.
You can also take this further to convert it to use Selgin's 'New improved head turn technique' but that is a more advanced lesson for another time.
part1:
part 2:
part 3:
Pt 3 is less about shape staking, but shows a simpler eye setup.
I also discuss Ramon's Lost Layer tool, and Synthsin's mod of Fazek's meshinstance script (I didn't realise Fazek created this originally hence no mention on the video, but Synth's version mod is the one I use)

Scripts mentioned in this tutorial:
Rudiger's mod of 7feets mod of the select shape tool (FREE - shows stack number)
http://users.on.net/~alexical/Anime%20S ... v6.0.2.zip
Rudigers premium select shape mod (highly reccomended)
viewtopic.php?t=18651
Fazeks tool replacements (couldn't live without 'em)
viewtopic.php?t=3507
Ramón López Lost Layer tool *pt 3*
viewtopic.php?t=16862
Synthsin's MeshInstance script mod (originally by Fazek) *pt 3*
http://lostmarble.com/forum/viewtopic.php?t=12024
* Synth's version has a better naming convension, but doesn't support curvature like Ramon's version - i've merged them to get the best of both worlds: http://www.mediafire.com/?1d7i66mtk8ck7ba *
in pt 3 I also mention Selgin's New Improved head turn technique:
viewtopic.php?p=87542 *pt 3*
Let me know if you have any questions!
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disclaimer: this isn't the only or best way to rig faces, but it demonstrates some possible uses.
I've uploaded the anme file here: http://www.mediafire.com/?bvn3c1hmiiikd80 if you want to investigate. I've also put the mouth masking shapes in their own layer so it might make moth animations easier (brings you small one step closer to Selgin's 'new improved head turn' technique

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here's a cartoon mouth using the same ideas:

I forgot to mention, if you want to select a shape that is below another select the top one (the one you can see) and control + down arrow to layers below it with the shape select tool...
if you put a mouth like this into a switch layer, and turn interpolation on, you can duplicate it and create all your mouth shapes - and it will animate between the mouth shapes you create!