As last minute tips are a plenty on what to do and what not. I will not delve into those things now. I will tell you what you can do to push your self and make sure that you can give your best in exam.
Make sure you consume solid food and not fruit juices. Banana, Apple is okay, but in solid form. Empty bladder before you enter the exam hall, even if you feel you do not have to. As you have to maintain a high level of focus throughout the exam and your eyes and brain can focus only so much, try to relax. After every 25 to 30 minutes close your eyes for 5 seconds and take a deep breath. This will release tension. Give rest to your eyes, brain and increase oxygen in body. You might feel these are small things but let me tell your such things like fatigue in an exam where all are competing for a few seats can cost you dearly in terms of silly mistakes (a miscalculation here an error in reading some portion of RC there and the list of errors can go on).
Do not drink water in big gulps. Take small sips of water every 10 15 minutes. This will make sure your body is hydrated. Carry a bar of dark chocolate (not milk solid) and consume it right before you enter the hall. This works as a good stimulant and makes the brain cells work faster (or at-least releases happy hormones - reduce stress levels).
All are unique and do not compare yourself with others. Relax and give it your best shot. CAT is not the end of the road in life. Life is long and you should have fun doing what you do. My facebook and twitter handles are in the home page, you can reach out to me for any help in your GD PI preparation for IIMs / FMS .
Best of luck.