There is no substitute for taking GMAT practice exams. Check your local bookstore for books and CD-ROMs of practice exams. Kaplan and McGraw-Hill both publish GMAT preparation books and run websites where you will find practice exams for a fee.
Practicing has several benefits. You will become familiar with the test format and the kinds of questions asked. Your practice test scores will highlight your strong suits and the areas you need to work on. Since you can't expect to know the answer to every question you encounter, smart guessing is a critical part of taking the GMAT. Practice exams help you get a feel for which answers to gamble on.
There is no magic number of practice exams to take, but you can gauge your progress by tracking your test results over time. Check the admissions information for the programs to which are you applying and find out what the median GMAT scores are for successful applicants. Aim a little higher than the median and keep practicing until you comfortably get into that score zone every time. Of course, the earlier you sign up for your GMAT, the more time you will have to space out your practice tests and avoid cramming.
The GMAT has three sections: quantitative, verbal and analytic writing. The quantitative section tests arithmetic, algebra and basic geometry. The verbal section tests reading comprehension, reasoning and your ability to spot and correct grammar and spelling errors. For the analytic writing section, you will complete two essays analyzing a topic presented to you during the exam.
When taking your practice tests, pay attention to the things that are dragging your score down and invest extra studying time into those areas. Some of the GMAT practice test materials include sub-tests for working on specific subjects. Your old college textbooks can be a good source of information to brush up on math and grammar rules.
Analytic writing is a little trickier to prepare for. When taking practice exams, don't skip the writing portion. Read your essays to see if you have really answered the question and if your essay has a thesis that is clearly argued and a conclusion that sums up your thesis and supporting points. Feedback on your college papers is a good source of information about what you do well and what needs some work in your writing.
GMAT prep courses offer more than just extra practice tests. The instructors can help you brush up on your weak areas so you can increase your scores, plus, they can give you tips on taking the exam. If working in a classroom setting isn't ideal for you, most test prep centers offer private instruction options as well.
Kaplan is a big name in test prep classes, but there may be other options available locally. If you can't find a test prep center in your town, an online course might be the right choice for you. Kaplan and Princeton Review are popular choices (see Resources).