The best way to prepare for the GRE is to simulate your testing environment and take practice tests. You can buy books containing multiple practice GREs, or you can save money by checking older editions out of the library. Most of these books thoroughly explain each answer choice. If there are any topics on which you feel particularly lost, the beginning of most test preparation books take you back through basic math skills and build up to what you need, textbook-style. With these books, you can go back and forth between the explanatory text and the sample tests at your own pace.
Similarly, there are many sample quantitative GRE practice tests online. Some of these are free, and some are not. Be sure to check the validity of the source; well-established brand names are always the most reputable.
The most expensive preparation option is the in-person GRE preparation course, offered by several companies. These are helpful if you lack confidence in your math skills, as the instructors regularly assist people through this struggle. An in-person course is also useful if you tend to procrastinate, since there are regular homework assignments on a weekly basis.
The quantitative GRE covers the early part of high-school math. Topics include ratios, geometry, algebra and problem-solving. Moreover, these topics are addressed as word problems, so be sure you know how to handle math as used in everyday life, and can translate the "language of math" into English.
The exam is computer-based, and you will have 45 minutes to answer 28 questions.
Do not wait until the last week to start studying, especially if math is your weak point. You will be much more relaxed if you start preparing a few hours here and there starting a few months out from the exam.
Since the exam is multiple-choice, if you run short on time, it is sometimes helpful to quickly estimate the steps of a calculation to eliminate a few wrong answers. This often narrows the possible answers to 2, giving you a 50 percent chance of choosing the right answer in only a few seconds' time.
New questions and question types have been gradually added to the GRE since around 2008, so older study materials may not cover everything you need. Additionally, the GRE is scheduled for a complete overhaul in 2011.