I recently concluded that part of the reason I have trouble drawing faces in manga style is that my concept of the 3D shape of the (simplified, manga-style) human head is somewhat fuzzy, so I decided to practice it a bit and see what I came up with.  I think I'm making some progress, but if anyone could pick these sketches apart, I'd appreciate it:

http://i.imgur.com/UxBVD.jpg

After that, I tried applying this to a sketch of Sara:

http://i.imgur.com/ucXRd.jpg