Writing Skills for the GRE & GMAT (Peterson's Writing Skills for the GRE & GMAT Test)

This book helped me score a 5.5 out of 6 on the writing portion. When I first began studying fot the GRE, I was intimidated by all the elements of the writing section: a vast pool of vague topics, impossible time limits, and my own cluttered opinions. I found Peterson's guide to be a practical refresher on rhetoric, fine tuned for maximum performance on the 45 minute and 30 minute essays.
My first few practice essays were rushed and unclear. On the day of the exam, I found myself with EXTRA time to edit and even add an extra paragraph. This could not have happened without Peterson.
I located this book at the public library a few days before the GRE- one of the best preparation decisions I made. It prompted me how to use supporting examples without letting them become cumbersome, develop a nuanced but clear line of reasoning, and skillfully deconstruct the arguments given in the argument-analysis portion.
This book might be a few years old, but its contents far outperform Barron's 2006/7 section on the GRE writing portion. I recommend using Barron's for everything else (verbal, quantitative tips and review, and the practice tests)- but complement your study with Peterson's book.
Also, I would recommend checking out the A&E video series "Biography of the Millennium: 100 People, 1000 Years" for an overview of western culture and history. It supplies some great examples to use in your essay.