Study, study, study! Don't cram the night before -- start at least a few months before the actual exam. Use GMAT study guides, flash cards and textbooks to brush up on the skills you may have missed or forgotten from your undergraduate years. You can study alone or with a friend. If you're willing to spend a little extra money, you can take a GMAT prep class, or get one-on-one tutoring.
For the math section, do as many practice problems and questions as you can, in all the different areas (data sufficiency, basic math computation, squares, exponents, etc.). Practice may not make perfect, but it will definitely increase your odds of scoring higher on the test. Keep going over the problems until your math comprehension score is slightly higher than what will be expected of you on test day.
For the reading section, you'll have to read large (and often very boring) sections of text. Practice reading passages from books, magazines and newspapers until you find a speed that is quick enough to move through the material within the allotted time, but slow enough to ensure that you've absorbed what you read. Once you've read a section, ask yourself questions about it to see how much you understood, and then go back and check yourself.
Also practice taking the essay portion of the test to make sure that you can construct a well-written, clear answer to the types of questions you will be given.
Don't just study the questions you'll be asked during the test. Also familiarize yourself with the directions for each section, so you don't have to read through them again and use up valuable test-taking time.
Finally, take a computerized practice test. It helps to take tests prepared by different companies, including the official GMAT company, GMATPrep (mba.com), and a couple of other test preparation companies. Try to simulate the actual test-taking environment as much as possible. Sit in a quiet room and time yourself. Take the same length breaks that you will be given during the real test. Score the test, and then focus on areas in which you didn't do very well, starting with your weakest areas. Practice those skills until you feel comfortable with them. Take the practice test again (and again) until your scores improve.