I just took mine this month (yay!) and can highly recommend the books that include the practice test CDs, like the Barron's books. I borrowed a really nifty little book that was a breakdown of many mathematical tips and vocabulary words, which was also a big help (
Barron's Pass Key to the GRE). Any time I was stuck on a question during a practice exam, I'd write down what type of problem it was, like "multiplying fractions", and then look up the tip. I could do geometry alllllll day, but couldn't for the life of me remember how to multiply fractions.
And I
owned that test.