Here, I am an American. So, I can help guide you with what you need to do. The key is practice, practice, practice. I would giver yourself about 3 months to study consistently. If you plan to go over that, you wil burn. out. I did twice. So, pick up these four books...ETS 10th Edition, Princeton Review Cracking the GRE, the Barron's book, and the GRE Math Bible.
Part of the way to improve with math is through repetition. They can only throw so many types of questions at you and you have to know the material cold. Kaplan and the Princeton Review don't tell you how to solve these problems analytically. They just have firedrill methods to get your score from average to above average. Worth through the Barron's guide first. This is pretty rigorous in both verbal and quantitative. I think it would take about 3-4 weeks to do that. Then, work through the GRE Math Bible. This has hundreds of problems to practice on. Once you make it through this book, the other problems are easy.
For the verbal, I would remember the Hit Parade and Beyond the Hit Parade in the Princeton Review. The Princeton Review book will help increase your efficiency. Although there are analytical methods to solve every type of problem, sometimes the quick and dirty method is better.
Barron's will give you two tests and the Princeton Review will give you a couple as well. You can download two more from ETS for free. With about a month left, I would start making my way through the ETS 10th edition. You will be reviewing real problems from the GRE. The math questions are easier than what you will have seen before. With about 2-3 weeks left, download the Powerprep CD and work through the problems they give you. Memorize problems because ETS does test from that bank.
Each week, take a test. This is where you will build the endurance and your time managment. I think this would drastically increase your score over 2.5-3 month period. After 3 months you will be exhausted and sick of the GRE. So take it. If you need to take it again, just think of the first one as a dress rehearsal.