Doing an edit off of Rydin's edit now, but I just noticed something-
21 colors? How the hell did you manage that? .jpg?
On the original and edits, I think you're missing out on a lot of depth, and overcomplicating the shading by adding wrinkles. (Wrinkles are great! Just don't forget the planes of the face.) Bengoshia did a great job, although taking it in a different direction. Pay attention to his lighting and shading.
Edit soon to demonstrate. (Am I coming off assy by critiquing edits? Sorry if so, it won't happen again.)
EDIT
Crits, then I'll show the edit.
Mostly for the mouth
(which I was too scared to edit.) When I make that face, pulling what you've drawn now is very hard. My top teeth overlap the bottom ones, so you only see the top; my lips make an upside-down 'D', not the '0' which you have. Also, study lighting. Faces are especially hard to light. Just find some refrences. Well done getting this far without one.
I've tried it, but historically I suck at drawing lips/mouthes.
Help me on this please, do point out any errors.
Anyway, here we go!
Ta daa!
Left the rights side as it was for contrast/because I was lazy. Take what you will from my edit. Added three colors, quick color reduction:
There you go.
Later,
Shrike