Figure out what your target score is. Ask the admissions offices what GRE scores are needed for acceptance into a program and for scholarships. If the info isn't published on the websites, call or email to ask.
Keep in mind that your percentiles in math and verbal are more important than the GRE composite score. Someone who scores 600 math and 600 verbal is in the 48th and 85th percentiles for math and verbal respectively. Someone who scores 650 math and 550 verbal is in the 61th and 74th percentiles for math and verbal respectively.
Even though both test takers have a GRE composite score of 1200, the second test taker's percentiles look better, over all, to an admissions committee. (source: ETS Bulletin)
Keep in mind that you cannot pick and choose which scores to sent to schools. Each time you retake the GRE, all of your scores from the last 5 years will appear on your score report. Graduate schools may either average them, consider only the highest, or consider only the most recent.
To reduce the chances that your score decreases in one area, make sure to study BOTH math and verbal between GRE retakes.
You can only take the test once per month and you cannot take it more than five times a year. To ensure that you can achieve a higher score on your GRE retake, give yourself about two months to study the material that you most struggled with. Consider buying new study guide and working with a tutor to improve your score.
Be sure to take a few full length practice GRE tests so that you can improve your endurance and your speed. The scoring penalty for not finishing the test is worse than the penalty for answering questions incorrectly.
Remember that the more you study, the more you can improve your score.