The girl's body is sort of... blocky. You can almost draw a straight line from the bottom of the picture to her stomach region. Try making her arch her back a bit more and make her a little curvier. She also seems to be sort of awkwardly stuck between profile and side view in some places, stomach region in particular. If I'm looking at the picture right, her arm is completely covering her left breast, giving her upper body a weird half profile, half side view look.
Good job though. And don't worry, it's a pretty tricky pose to draw realistically. I couldn't do it half as well.