Every GMAT data sufficiency question will have the same five following answer choices:
A) Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B) Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C) BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D) EACH statement ALONE is sufficient.
E) Statements (1) and (2) TOGETHER are NOT sufficient.
Rereading these choices during every data sufficiency question will waster valuable testing time, so make sure you know the answer choices cold. If you have trouble remembering which answer choice is which, try using the acronym 12TEN - A) (1) alone, B) (2) alone, C) Together, D) Either, or E) Neither.
Because you take the GMAT on a computer, you will not be able to mark up the problem as it appears on paper. Having a solid notation system to use on your scratch whiteboard paper will help you keep track of how you've worked through a problem and the answers at which you've arrived.
For data sufficiency questions, try writing a large 1 and 2. As you work through each statement separately, circle the number of a statement if the statement is sufficient and cross out the number if the statement is insufficient.
Then, you'll know whether you need to work through the problem with both statements. Make sure to keep your work for each problem separate and easy to read, too--jumbled scribbles are only going to confuse you and hurt your score.
Remember, data sufficiency questions are asking you whether you can solve a question, not for the solution itself. Solving a question can be tempting, because arriving at a solution is a sure sign of a sufficient statement, but going through and solving every question is a waste of time. Once you know that a question can or can't be solved, select the appropriate answer choice and move on.
The GMAT quantitative section requires you to answer 37 math questions in 75 minutes, giving you just over two minutes to spend on each question. If you're stumped on a particularly difficult data sufficiency problem and it's taking too much time to solve, consider guessing and moving on. Getting one difficult question wrong will not hurt your score nearly as much as failing to answer all the questions in the section.
Although there are plenty of example data sufficiency questions out there that have been written by test prep companies and tutors, for the best preparation, stick to prepping with questions that have been written by the makers of the actual GMAT. You can find these problems in the published Official Guides and the GMATPrep software that comes with every test registration.