I used the Kaplan books and raised my score by 300 points using them! After working through all the problems, the GRE seemed easy in comparison. Since I am an older applicant, I had been away from math for a long time, so Kaplan especially helped me in the quantitative section. I didn't have to improve much on the verbal, so I concentrated mostly on the math. I didn't use any other source, so other practice tests may be just as good, too.