First of all, start preparing for your exams early, at least a month before your exams are going to start. In this way, when your exams arrive you would be so well prepared that your only tension would be not to forget your stationary at home. ;) If you, however, start only a couple of days before the exams, you will have to cram everything within two days. Since that can never happen, you are left to deal with each subject just a night before it actually is. This causes tension in two ways: First, you are worried if you will finish your course in time and secondly, since you have to stay up late to finish it or leave it and go to sleep, you are tense during the exam if you would know the answers or not. So, start early and do not leave all the studying for the last days.
Also, do not set over-ambitious goals for yourself. Do not make a study timetable that commands 12 hours of study every day. You just would not be able to do it and would fret about not doing it even more. Be realistic when you make your study plan. You should not compromise on your sleep, exercise or diet.
Then, plan small breaks between studies. Do you have a best friend who would want to support you? Make your study plans together and take breaks together maybe in the form of short 5 minute phone conversation with each other. May be you can take a smoothie break, go for a short run around your living room, do a little yoga or even just go and give your parents a hug. Just do not take too many breaks and do not take breaks that are too long.
The biggest No that I can think of is sleep deprivation. Get proper sleep, at least six hours. That way you are less jumpy and you learn faster.
Once you have the exam paper in front of you, you should tell yourself that you know everything you did in class and that you have done your best. Then open the exam paper. When we start by thinking that we do not know something, we often forget that we actually do know it. Also, if there is a question you do not know, do not panic. Move on to the next question and at the end of the exam, come back to the question and solve it the best you can.
I hope these tips help you and I think you will do very well! Good luck! :)