With the wealth of GMAT prep materials out there, it can be tough to find the best resources for GMAT study. I've been tutoring for the GMAT for five years, and these are the materials I've found to be the most helpful.
GMAT General Study
The best source of practice problems is The Official Guide for GMAT Review, published by GMAC. Working through the questions in this book will prepare anybody for the questions on the test. GMAC also publishes practice tests that are just like what you'll see on test day, and they are a great way to gauge your current score and get used to the CAT format. The software, GMATPrep, is available for download from mba.com. For students who need more practice problems, I recommend The Official Guide for GMAT Quantitative Review and The Official Guide for GMAT Verbal Review. These books offer additional practice, but the problems are not as difficult as the problems in the The Official Guide.
GMAT Math Study
For students who need to brush up on basic math concepts, I recommend The CliffsNotes Math Review for Standardized Tests. This book breaks down all the basic math concepts a student will find on the SAT math section into small, easy-to-understand chunks of information and provides plenty of focused practice problems to drill home each idea. Thorough study of this book will help any student fill in gaps in math knowledge and strengthen any weak areas.