What really helps is breathing properly before you walk into the exam room and during the test itself - it's not trivial stuff, breathe in through your nose for four seconds, hold it, then out slowly. With practice exams instead of seeing them as tests, use them to figure out your pace and where you're losing the most time, so when the actual exam comes you'll already know how to approach it and your head won't be in full panic mode. If you feel panic rising during the test, stop for two minutes, read the question again and remember that some tension is normal and actually keeps you sharp - the real problem would be having zero adrenaline at all.