Edit: Feel a bit bad and I guess I can help from doing my GCSEs last year?
Basically make sure you know your stuff, if you don't, don't stress out in trying to fit it all into your head a few minutes before, because it just won't work and you will panic. Make sure you are completely prepared with what you need, as in pens, pencils and stuff, and make sure you are on time. If you don't do this you will start panicking and probably won't perform to your full potential. @Harry Brown
Go for a 30 minute run tonight so that you are knocked out throughout the night. Just chill out and don't panic. Cook yourself a nice breakfast in the morning too!
Make sure to have a good night's sleep, helps a lot. If you haven't revised fairly well in the past you should probably stay up studying to half-past ten or so then sleep.