Decipher the directions. These may be confusing and your best approach is to make sure you understand them before test day. There are many websites that quote the directions verbatim--make sure you understand what you are supposed to do.
Review basic math. The GMAT data sufficiency questions cover information from algebra, geometry and basic arithmetic: averages, fractions, decimals and percents.
Take a class. Exam preparation classes are available online and at local universities, sign up for one to ensure you will succeed on the test.
Practice. Whether you complete a GMAT review book (available at bookstores nationwide) or do practice questions online, these questions will become much easier with extensive practice.
Breathe. Take a minute to consciously relax before you begin the data sufficiency section of the GMAT. Remind yourself that you've studied for the exam and you're ready to go.
Read the questions very carefully. Use only the given information. It is easy to assume information that is not included in the question, this can lead to mistakes.
Eliminate wrong answers. Once you rule out answers that are clearly not sufficient, you will have fewer choices for the right answer.
Examine each of the statements individually. Ask yourself if either of them are sufficient to solve the problem or if you need to use them together to find the solution.