Retro Sci-fi helmet

I have only one suggestion: and that is to use a Specularity map.

When you' talk to makeup artists, you'll eventually hear them talk about the "T" zone, which is the bridge of the nose and the area above the brow. Typically, this is shinier than most of the rest of the face.

What I do is use whatever bump map I have, darken it and jack up the contrast, then overlay that t-zone in white as well as some other raised parts of the face (Chin, Cheeks and Lips).

Apply this in your specular strength and you'll get more realistic looking skin, especially if you use a light that is spec-only.