Close your eyes and think of something peaceful and enjoyable for half a minute or longer. It could be anything from lying in a field watching birds, standing by a waterfall, floating in the sea,quietly cuddling someone you love or watching children playing happily. Make it feel as real as possible. It will calm you down and lower your heartbeat - indeed I sometimes use this technique when having my blood pressure measured at the doctor's.
It's called "Guided Imagery" and it really works - I often used to do it before a court case, to still my mind.