I just took the GMAT this August and I used the Princeton Review "Cracking the GMAT" book to prepare. I found the math review very straightforward and useful for re-learning the math that I had forgotten from high school and college. Also, the GMAC people publish a book with practice problems. I found that helpful because beyond learning the math, I needed practice drills to get used to doing them quickly and guessing when I didn't know the right answer. I ordered both off Amazon. Hope this is helpful.