Hello, well this is what i do for anime and it works very very well and it's very good for looping...
(it's my own standard...)

when it doesn't look right i just follow the sound waves...

i use 3 different mouths..

O = mouth wide opened(as much as you actually
want.... bigger if screaming)

o = mouth halfway opened

- = mouth closed or simply showing teeth...

well, here is how it goes.... (keeping the 12 fps setting)

- - o o O O o - - - O O O o o O - - O O o O O O o o o -

make the last frame go back to 1st frame if you want it to loop...