Rendering and sketching wise you got some decent skill going on. There's a few things I think you'd be better off working on:

-Conveying material: A lot of it except the hair seems like its made of the same stuff. Look at some references and try to get some small details in there.
-Defining a lightsource: Even though this is more of a pin-up/presentation piece, you still gotta decide on a proper lightsource. Sure, some have shadow, but a lot of the shadows contradict one another and others should be in complete shade.
-Don't get caught up in too much detail: Biggest offender is the hair. That stuff dont need every individual strand shown, maybe some studies would help?

Overall man, you got the tools to move forward. Looking forward to see more.