Hey Danielle
I'm in the same boat as you.
My scores are ranging from 630 to 650, but what really improves your score are some key things that I can list below:
1) Analyze your errors: after a lot of time spent, I was repeating the same errors again and again. Try to list what your weaknesses are, to defragment your thinking pattern during the exercises, and see where you do get "stuck";
2) Dominate the fundamentals: probably some key issues of your weaknesses is that you did not have the fundamentals dominated. As the St. 1 in this list, if you do discover a weakness in some area (geometry, for example), review deeply the fundamental concept of that area;
3) Quality practice: doing 354 thousand exercises of some underground gmat prep will not help you - it will just reinforce that wrong pattern of your thinking process. Try to select the best materials and go for it;
4) Motivation: probably one of the main points, one of the most critial, is that you need to keep yourself motivated to your goal. I do myself feel exhausted (work for 13 to 15h per day), then reading about combination problems is not the first thing you want to do. Try to relax sometimes, or you will get even lower scores and you sometimes experience brain meltdowns (talking about personal experience!).
Hope that helps.
Roberto