initial thoughts: - The window is still closed, and there is no physics to her hair...

everything else in the photo has atleast some level of blur, maybe play with her a little bit, she is too in focus.

Yup,I agree with Keiley22, adding some blur into a woman would make it perfect and more realistic. Good luck author.

I disagree, it's not that she is too in focus, the car has been shot using a high shutter speed, and what looks to be tracked judging by the slight motion blur to the BG. So she would be just as in focus as the car. There is also an obvious difference in the lighting and the amount of noise between the source image and the stock.

So

1) Colour correct her to match the stock (use curves)
2) Add a reflection to the car of 'her' left arm and drop to opacity right down
3) I think you forgot to apply my previous advice to the part of the window under her arm :P and darken those areas even more! also, there is a pale line just where the window meets the 'car' - paint that out.
4) Apply some movement to her hair (either find another stock image, which might be easier / or paint and manipulate the source image)
5) It might be easier to get rid of her necklace than to make it match the scene.
6) It might not work but add a layer of noise and bring the opacity down - just an idea.

thanks all for suggestions ..