Here's a paint/manip of my cousin (who has wonderfully geeky sensibilities and is a good sport.) Crits appreciated.
Honestly it's already pretty cool design wise. I think you could perhaps push the form, but i guess your lighting situation is on a cloudy night with a lot of scattered source light? Regardless i think you could push and pull some of the geometry around the mouth, and maybe indicate some textural difference between the dry skin and maybe the gums and teeth with some speculars. Also maybe have the nose set back into the skull more as if it's more of a skull, but perhaps that may be more ghastly than your going for. Also maybe define the hair a little more and your on to a winner. Good luck with it.
The pice is quite flat so you should work more on the form definition of the face. Furthermore there is a big lack of shadows as it seems to be night. In general I would raise the contrast a lot more. Than you could play with some more details to bring out the rotten zombie skin. Some infections, wounds, bubbles and so on. Dont just put a few phototextures. I recommend you to think more about the nature of the surface you are working with. How does it react to light, is it wet, does it reflect,.. The eyes for example dont make sense. They are neither wet nor dry and dead. Search for reference! The practise idea is funny!