The HTFT recently gave some great advice to students siting exams. Having confidence in the work you have done is key, but it said you should not revise for more than 45 minutes at a time.
Try to split your revision, with five to 10-minute breaks every 45 minutes. Creating topic summaries and revision cards are also a good use of your time. You could then get family and friends to test you.
Finally, some advice that is a bit more out there – it said some people swear by Magnesium tablets to reduce stress.