ok basic relaxation exercise:
no shoes, comfortable clothes. lay down flat on your back, arms at your sides. Starting at your tose tense every single muscle in your feet. Keep them tensed while you tense your legs, then your stomach, then arms and finaly neck. Stay like that for a few seconds.
now, slowly, again starting at the toes, relax the muscles. let them go totaly limp. slowly work your way up your body in the same order as you tensed. concentrate on how it feels when you release the tension. Not only will this help you train yourself to be able to actualy relax at a moments notice, it's a great thing to do just before fallijng asleep at the end of a long day.
Dont just run through this once though, like anything else you have to repeat the actions enough times that your muscles learn and no longer depend on your brain to tell them what to do.
I used to do this one and I want to add something to it. After you tense the muscles and move into the relaxing phase say a phrase (mentally is fine) like "relax now". This anchors in the concept of relaxing and gives you something specific to use when you are tensing up. The phrase will bring back the relaxing.
What I do now when I'm getting really tense is a variation of Transcendental Meditation. I go lay down for 15 minutes or so and mentally repeat a phrase over and over. It sometimes produces phenomenal results.