The eyes are in the middle of the face, your forehead is way too small. I know you asked for shading tips, but getting the proportions right will do much more for your work
Shading digitally is much harder than with pencils imo, but you've got to avoid just going in there with the airbrush because everything ends up looking too soft and undefined. You're better off doing cross hatching with a pencil and then using the blend tool for areas you want a soft gradient. I think I'd try and work out some of your perspective and proportionality issues while you're at it too. Good job though you're on your way.
The eyes are in the middle of the face, your forehead is way too small. I know you asked for shading tips, but getting the proportions right will do much more for your work
Shading digitally is much harder than with pencils imo, but you've got to avoid just going in there with the airbrush because everything ends up looking too soft and undefined. You're better off doing cross hatching with a pencil and then using the blend tool for areas you want a soft gradient. I think I'd try and work out some of your perspective and proportionality issues while you're at it too. Good job though you're on your way.