There are a few convenient tricks you can use to convince your body to calm the heck down. A slideshow at Health.com shares five fast solutions.
You can stare at the ceiling and count down from 60 slowly in order to lower blood pressure. You can write down your worries in a notebook and set it aside until tomorrow, which in the interim might allow your mind to think about other things since your worries have been properly documented. Breathing in through your nose and out through your mouth 10 times slows heart rate to calm down the body. As a meditation tool, imagine using all five senses what it would be like to float on a cloud (yeah, okay, even I roll my eyes at that). Lastly, in order to reduce muscle tension, you can slowly flex and then relax your muscles, starting at the top of the body and working your way down.
You can view the full slideshow here: http://www.health.com/health/gallery/0,,20502271,00.html