Study other GMAT exams and pay attention to how each question on the test is a data sufficiency problem. Data sufficiency means that all of the information you need is given in the question: There are no trick questions on the test and "none of the above" is never a valid option. Once you realize that the answer is in front of you, it becomes a matter of logically working through the problems.
Work as many real-life data sufficiency problems that you can get your hands on. Plenty of practice will prepare you for the test. Figuring out a budget, your car's performance in miles per gallon or summarizing your stock portfolios are all examples of real-life data sufficiency problems.
Make recreational puzzle solving and game playing a priority in your free (non-studying) time. If you get bored with studying traditional math problems, try a puzzle book like one with logic problems or Suduko. Puzzle books are a great way to exercise the mathematical and logical portion of your brain. Play games that involve logic and strategy, like chess, Civilization or Small World.
Review your high school elementary algebra and geometry textbooks. If you don't have them, check your public library for books on those subjects. You'll need to know basic algebra concepts like solving single variable equations and common geometry formulas.
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