I think everyone covered many of the silly mistakes I have made over and over again. Instead of going over common math/verbal errors, let me say a few things about stress management. I recently took the GMAT and should NOT have done the following because I got psyched out.
1. Do NOT Drink coffee (or minimize)- your heart rate will naturally go up on test date, coffee makes someone like me gittery.
2. Do not talk to other people during breaks because they might say something about the last section that will put thoughts into your head and kill your confidence....try to avoid people (esp. ones you know) and stay focused
3. Bring a timer to time your breaks becuase you want to get as many minutes as possible without being paranoid that you need to go back as soon as you return from the bathroom
4. Do not think about all the people you have to answer to once you are done with the test - it only adds pressure
5. Eat sugary stuff in between breaks to give yourself a boost.