Using switch layers is the easiest way to change facial expressions, in my opinion. I have been able to rig some facial expressions using bones, but it can get complicated.
My recommendation:
Make separate switch layers: one for mouth and one for eyes. Make the first vector sub layer with eyes open. Include eyelids in the vector layer. Of course, you'll want the eyelids to be the topmost object (see tutorials for layer ordering).
Since the switch layer is also a bone layer, make bones to bind to the pupils of the eyes. Genete's springy mechanism tutorial shows a useful bone rig. using two springies together, you can manipulate the eyes left, right, up, down, diagonal.
After rigging the eyeballs, use the copy layer button (the eyes will already be bound to the bones if you did so before copying).
with the new layer, make modifications for the facial expressions you want: partially closed, fully closed, wider open, etc. Repeat for as many expressions as you want. You can always add more while animating if you need to.
Make a switch layer for mouth, then a vector for a closed mouth. For most characters, I include the chin in the mouth drawings so I can make the chin move while they speak. I'll make vector layers for standard speech syllables, so I can use them in lip sync. Modify each vector layer for the expressions you need.
Springy Mechanism tutorial:
http://genetita.googlepages.com/videotutorials
