Go further with brushes
tip 1 hard erase
While the whole illustration is painted using Soft Brush, areas such as eyelids and nostrils are being more clearly defined than others by hard erasing or hard masking, which helps to create a sharp contrast.
tip 2 soft masking
Further contrast can be added around the nostril areas by soft masking the nose shadings to create a slight highlight on the nostril edges against the darker shades.
tip 3 Create textures
The soft textures on the forehead can be created using a slightly blurred noise layer with Softlight blending mode, using a Soft Brush to unveil the textures through masking.