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.