Chances are, you figures have very weak scuplting in the face, so you're at a disadvantage. Replacing the heads with some from the Warriors aftermarket sets will go a long way, not only better sculpting, but more expressive, as well.
As to the painting, do a search here, at http://www.missing-lynx.com/ and at www.armorama.com for articles on painting faces. Then practice, practice and practice some more. Try the diferent styles you'll read about. Try oils or Andrea/Vallejo acrylics as your medium.