Relaxing during training is something that I have struggled with for a long time. I agree with all of the above...especially...breathe, breathe, breathe! As you exhale, feel your shoulders relax and drop, feel your abdominals relax and drop, feel your butt relax and drop. Your legs carry your weight and should be relaxed too (don't worry, they'll hold you up) and you should feel your weight in the bottom of your feet. I have been told that eventually you can feel your weight in the ground below you...I'm not there yet.
As far as "practice crap, perfect crap" ...Oh so true. Try doing your forms at what a friend of mine calls a "Baby slow" pace. Breathe with every movement and make each movement slow, steady and without end; in other words, one movement should lead directly to the next. Be sure that you feel your muscles relax until the moment of impact and then relax again. Do not stop your motion at impact...relax, hit, relax while continuing your motion. There's no need to tighten your entire body while doing this exercise. Only tighten the muscles you need for the moment of impact and only for that super brief moment. Remember, power is mass plus velocity NOT mass plus tension! With tension there is no velocity.
Even if you are in a hard style you can practice forms in this manner (with the exception of Okinawan sanchin which must be performed with every muscle tense and with a very specific breathing rythm). This slow method of training with correct breathing and concentration will (in my opinion) help you learn to relax. Good luck!