Thanks for your input guys, I really appreciate it. I will work on the things you suggested.
It looks like I'm on the right track though, because I have already been doing stuff like this. I've been really working on getting her into frame and working her hind end. Her butt muscles have come such a long way since when I got her a year ago, and her overall muscling has improved so much. I'm hoping the same will happen with her topline.
Also ... its hard with a thoroughbred too, because they have high withers and pointy croups.