Try relaxing every group of muscles, starting with the feet going right up to your head. I visualise them sinking like lead weights through the mattress and then floating like balloons above it...once you have dealt with your whole body, try deep breathing, in for 4, hold for 4, out for 4, hold for 4......no matter how stressed, I am usually out by the second set of deep breathing BUT if that doesn't work, after all that regime then try counting backwards from 100, then 99, then 98 etc.

It's very basic relaxation and self hypnosis but it works. At the very least, you are concentrating on the regime hard enough to displace the 'busy work' that is going on in your head.