You pretty much do it the same way you'd adjust curves for joints or other objects. Say you have facial blendshapes, you select the original face geometry that has the blendshapes, and open your graph editor... you should see the curves that make up the blendshapes in action
I lttle tip I got from Paul Mikulecky of Lost Pencil www.lostpencil.com
was to keep curves for facial blendshapes in spline mode, makes the face more natural.
Hope this helps,
God Bless you and 3D