It took me a little less than a month to prepare for the GRE. I didn't really focus too much on the math section because I have always been good at math. As an undergraduate, I majored in Microbiology and Chemistryand got a minor in Biomedical Physics. My curriculum was very math instensive, so when it came down to taking the test everything was a review. I did all the test problems in the official GRE study guide and that was enough for me.
However, I was surprised that during the test day I was struggling with some of the math test questions. I had some hard math problems and it took me a while to finish. I was definitely sure that I didn't get the elusive 800 in the math section that I wanted so badly. However, I guess I got most of them right and the test was only hard because it is computer adaptive. My advice would be to not get discouraged if you find some the test question difficult and just focus on getting most of them right.
My major concern was the Verbal section. My advice on doing well on the test is to increase your vocabulary. I did this by learning the words that are in The Princeton Review Word Smart for the GRE, Barron's Essential Words for the GRE, GRE Bible's wordlist, and by reading novels and looking up unfamilar words on thefreedictionary.com. I would recommend using thefreedictionary.com because it allows you to make your wordlist which makes it easy to review them later. Once I became confident with my newfound vocabulary I spent two weeks doing verbal problems. Despite working on my vocabulary, I still found some words on the test to be totally foreign like the word Friable :(.
I took 1 computer adaptive test from GRE BIBLE and did all the samples problems. I took 3 tests from Mcgraw Hill GRE 2009 CD. I thought the Mcgraw tests were easy but were scored much harder.
I was scoring in the mid 1400s on my practice test, so when i saw I got a 1520 on the real thing I was excited.