The big place to concentrate on relaxing is the little muscles right under the back of your skull (suboccipital muscles).
See this link for info on how excessive tension in these muscles will generally decrease your athletic performance:
This subject will be addressed much more in a Z-Health private session or a seminar than in you at home with a DVD.
You will have to reduce the weights you are using to practice this, likely severe reductions in weight. You've been practicing being "tense", whether you know it or not, so you'll have to practice "relaxing" a lot, to undo your old habits. Anything heavy will cause you to revert to your old habits.
Look at the faces of Olympic sprinters...they are putting out a lot of exertion, yet they are not scrunching their faces in tension. This is key to athletic performance and longevity.