It is important that you stay calm, healthy, and hydrated on the day of your GRE test. Make sure to not let anxiety get the better of you throughout the test.
Also, make a list of target universities, take all the documents and things that you need to take to the exam, and reach the test center early.
There are some very important things to do the day before the test, and on the day of the test as well. We've written about test day strategies in detail, here: crunchprep.com/test-day-tips