Here's another tip- Don't lose your cool during the test
(This being from a person who's never taken a GMAT, but I have taken similar test)
During the middle of the test, your brain will be toast. You will feel on some pages you are getting no more than 10 -20% of the questions right. DO NOT LOSE CONFIDENCE!!!!
These test are calibrated to throw out certain questions based on results- so yeah- maybe you only got 50% right, but the guy next to you only got 30% right- so in the end, you will win. I took national boards in a certain subject-came out feeling like a dumbass- in the end scored higher than 97% of everyone else in the country.
Bottom line- don't get flustered.