You should do more study of the female form in general. I found that doing traditional sculpting classes have really helped me improve my 3d skills. I suggest subscribing to www.3d.sk for reference images.

here is a more detailed markup for you to look at. One needs to remember the lines of the face and that the upper lip (unless one has an underbite) is always out further than the lower lip.

good start by the way =)