Overconfidence
Don't think that you can drop into the test testing center and cruise through the test with no effort. Think again. The test- if required by your chosen School - is important. Be forewarned: You should take it very seriously.
Focussing on some areas only
Some candidates will focus on strengths at the expense of their weaknesses, while others will concentrate only on their weak areas and neglect their strong areas. Both approaches are dangerous. Your test score will be based on how many questions you answer correctly and their difficulty level, but also on the range of question types and specific abilities covered by those questions. Prepare carefully for all sections of the test. Also note that Schools often have their own formula for weighing your individual scores. Try to perform your best on every section and every question type within each section.
Undue emphasis on practice-test scores
Even though your preferred School may have a declared minimum total score for entry, setting a goal for your test score is understandable. Try not to concern yourself as much with your scores but with what you can usefully do between now and your test day to improve your performance.
Over-preparation
Preparing for the test is like training for an sporting competition. Familiarize yourself with it and get comfortable with it. Build up your endurance. Aim to have your motivation, interest, and performance peak on the day. Preparation (training) e.g. getting comfortable with the test, correcting poor test-taking habits, developing an instinct for spotting wrong-answer choices and to finding your optimal pace. But after a time additional practice will give little additional benefit. So - don't over prepare by starting many months in advance or by postponing test dates to give yourself more time than you actually can usefully use need for preparation.
High expectations
In theory you are capable of attaining perfect scores, but you do not need to do so. Everyone is limited to some extent by their abilities. Accept your limitations. Prepare yourself so that you can do as well as you can reasonably expect.
The essays
Schools often clearly state their admissions requirements on Quantitative, Verbal, and Total scores, but they are often a lot vaguer about their requirements for essay scores. This does not mean that you can take the test essay sections less seriously! The top School for you will look at all the evidence - nothing in unimportant.
Negative views
Don't make the mistake of thinking - I'll give the GMAT one go, and if I do poorly, I'll not try for an MBA program. With time and effort you could do very well. See it as preparation for your study. Register for and take the real test once as a full practice just to get comfortable with the testing environment. You'll be far more relaxed the second time around. More than 90% improve their score in the second test.
more information: http://www.mbaprograms.org/mbaadmission/gmat2.asp#advice