Learn to recognise when you're stressing out. A break or a chat with someone who knows the pressure you're under will get things into perspective.
Avoid comparing your abilities with your mates. Those "I've only read Macbeth 17 times" conversations are such a wind up. Everyone approaches revision in different ways, so just make sure you've chosen the method that works best for you. Make a realistic timetable. Stick to it.
Eat right. Treat yourself like a well honed machine - eat fresh fruit and veg and have a proper breakfasts. Fuel your brain as well as your body - no one can think straight on coffee and chocolate.
Sleep well. Wind down before bed and don't revise under the duvet - your bed is a sanctuary, not a desk. Get your eight hours.
Exercise. Nothing de-stresses the mind faster than physical activity, so build it into your timetable. Being a sloth makes our mind sloppy too.
Panic is often triggered by hyperventilating (quick, shallow breaths). So if you feel yourself losing it during the exam, sit back for a moment and control your breathing. Deep breath in and out through the nose, counting to five each way.
Steer clear of any exam 'post-mortem'. It doesn't matter what your mate wrote for Question 3(b), it's too late to go back and change your answers, so it will just make you worry even more.
Ultimately, don't lose sight of the fact that there is life after exams. Things might seem intense right now, but it won't last forever.