I would second what Glasses said.
I just kept on taking the two practice test that ETS sends you when you register for the GRE. If you are the least bit mathematically inclined, I think that's all that's necessary for the quantitative section. It's more about knowing how to look at the questions rather than doing the math. This method worked to a lesser degree for the verbal section. The best way to improve in this section is learn the word lists. I just used the test as an excuse to read some novels, hoping my vocabulary would improve.
I went from scoring around 600/400 when I first took the practice tests to consistently scoring around 800/600. (I actually practiced with one test until I started scoring at the level I wanted to. Then I took the other test to make sure my scores weren't a result of becoming familiar with the questions). When I took the real test I scored 790/630 which is about what I expected going into it. So that's, a 400 point improvement just by familiarizing myself with the test.
I did by some books but I can't say that I read them at all. I also found the practice test included in the books are not very representative of the actual test. Generally they are more difficult and under report your score. If you have friends that have taken the GRE in previous years, see if they still have their CD with the practice tests on it.
Funny story...When I was taking the test, I accidentally kicked my computer's power cord when I was about half way done and the computer turned off. :shock: I was so scared. Luckily, the computers are set up to save your progress as you go. Still, my nerves were a bit rattled and I don't think the incident improved my score at all.