Taking the GMAT last year and doing quite well (I normally don’t tell anyone my score, but I’ll say it was in a top percentile), I found that there was really one thing that made a big difference in how I did.
I initially tried to study out of a book, but I will actually say with the conversion over to the CAT (computer adaptive test) version, studying out of a book is pretty useless.
Instead, you need to go out and buy or borrow from a friend a test prep CD (Kaplan or Princeton Review – though I liked the Kaplan CD better). Don’t worry too much about the lessons (though if you are having problems with a section they might prove useful), you need to go through every single practice test – every one. They have practice versions for each section and complete tests.
Taking the CAT is completely different than taking a book, in the feel and in the timing. You should know exactly how to use the computer before you walk in, and you should have a good feel for the timing. Because the test gets harder as you go on (if you answer the questions correctly) you need to know how much time you need for the easier and harder questions, especially since you can no longer go back and answer them again.
In addition, there are a few tips on the CD that will prove to be very useful. The one I found most useful was the 3:2 rule. In the English section, if you have 3 answers that are the same, and 2 other answers that are similar, 90% of the time the 2 are wrong and you can cross them out immediately.
That tip alone saves a ton of time, crucial when you get to the harder questions.
I will write another epinion pointing to the CD I bought, but I think that if you buy any one you’ll be ok.
Best of Luck!