1. Relax. You mood during the exam is the most important thing. Try to keep yourself under control. Don't get too excited.
2. If you haven't, do one or two test simulations before the exam day, following the exactly the same timeframe of the real one: 2 writings (1 hour)->5min. break->Quantitative (75min)-> 5min. break -> Verbal (75min). This might help you warm up and expose something you didn't know about such as your body energy consumption.
Hope this helps. Again, relax and try to enjoy the whole thing, it's all about experience.