How can I improve from low 600s on SAT Math?

I've taken all the practice tests in the SAT Blue Book, and I just keep scoring around a 630 on Math. I usually get thrown off by a question that I don't know how to solve and then that anxiety spills over into the rest of the section. I was really hoping to get my score up to a 700 on the next test, but it seems like I can't improve no matter what I do. Where do I go from here?

You should go through your mistakes and see if you can detect any patterns in the types of questions that throw you off your game. Identifying the specific problems you have is the only way to fix them. You might also consider getting other review books that provide more practice and detailed answer explanations. Some good ones are:

Dr. Chung's SAT Math
Barron's SAT Math Workbook

These books will give you strategies for dealing with the specific struggles you have on the math section. It also sounds like you have some issues with test anxiety, so you should try and get more comfortable with skipping questions that you don't quite get on your first pass through a section. Rather than letting a tough question bog you down and throw off your flow, just skip it and finish all the questions you know. This way, the problem question will have less of an impact on your performance in the rest of the section.