I feel that a lot of problems stem from a lack of knowledge in anatomy; notably the the lower half of the body (buttocks and legs).
The butt lacks form, it simply looks like two circles while in reality it is quite far from the case. Additionally, consider the fact you're using a counterpose (with the leg from our left being the weight bearer) and as such the butt cheeks will be at an angle, with the left one being higher and more stressed, and the right one being lower and more relaxed.
The left leg should have more definition in terms of musculature, to fit with the fact it's bearing most of the weight. The right foot should be pointed towards the viewer more than it currently is; while it's not impossible for it to be facing inwards as it is now, I think it would be more graceful if it wasn't.
I think that you should move the front elbow closer to the viewer, as it seems rather unnatural for it to be that close to the back (try to imagine how it would look from a bird's-eye view/above)
With the hair you should try to imagine it being twisted upside down (as if it were a piece of fabric for example) and wrapped around the shoulder.
Also, be careful with where you use the black; in my mind it implys the darkest spot you can have in that image, that is, the area with the least light being reflected.
This edit is far from perfect, but I hope it is of some help.