As the date of your GRE nears and your preparation strategy threatens to become a blur of practice tests and study guides, take a step back. Start by reviewing your performance on your latest practice test and identifying your lowest scores by section. If, for example, your verbal reasoning score lags behind your scores on the other sections, it makes sense to spend more time reviewing this area.
Begin by concentrating on the specific questions you answered incorrectly. Review them and determine the correct answer. You'll soon find out if something simple is tripping you up, such as rushing to choose an answer before moving on to the next question. If that is not the case, you may discover that you need to review an particular block of material.
Keep in mind that you may not need a GRE-specific study guide to review for the test. For example, most of the GRE math section is aimed at 9th grade (or lower) skills, which means that any exercises that focus on middle school math will help [source: Mendelson].