The computer-adaptive structure of the GMAT
The computer-adaptive test (CAT) version of the GMAT is designed to get a more accurate assessment of your skills while asking fewer questions than its paper-based counterpart. Here is how the test works:

The first question you will see in a given section will be of average difficulty. If you get the answer right, your next question will be slightly difficult. Conversely, if you get the question wrong, your next question will be slightly easy. This also means that the software will ask you different types of questions in a rather unpredictable order as determined by its algorithm.
You cannot skip a question or go back to an earlier question. Unlike the paper version, once you click the answer confirm box, your answer cannot be changed.

Managing your time wisely
Spend adequate time on the first 5 questions.
The more difficult questions are weighted more heavily than the easier ones.After first 5 questions, the testing software will give you questions to fine tune your score within a narrow predetermined range. Thus, your answers to the first 5 questions will make a huge difference in your final section score.

For this reason it is imperative that you answer these pivotal questions with extra care. Always double check your answers and verify that the incorrect choices are indeed incorrect. If you are unsure of one of these first questions, at the very least, take a very good educated guess using process of elimination.

Read the Questions Carefully
The undisciplined test taker feels the stress of the clock during timed sections and will try to cut corners to save time however-whenever possible. As a result, questions are often misinterpreted. You will encounter questions when you sit for the GMAT with incorrect choices that were deliberately designed to exploit a misinterpretation of what the question is really asking.

Avoid Random Guessing
You must select an answer before the computer will allow you to move on to the next question. Even if you don't know the answer, it is advisable to take an educated guess than random guessing. One way to do is identify at least one choice that is clearly wrong. Eliminating even one incorrect choice will improve your odds.

Practice, Practice, Practice
Practice using the actual questions from previous exams. Spend time studying and practicing questions in your weakest subject areas, as this will provide an efficient use of your test-prep time.
Should you need to retake the exam, you will need time to register again and have the new score submitted to the school in time. Waiting a long time into the admissions season to take the exam is also bound to cause undue stress. Plan properly and with insight and focus on maximizing your GMAT score.

For more detail: www.gmat.org