So I'm not sure there is such a thing as a "meta-skill" to be relaxed as a default state or to be able to relax at will. (Some people, like monastics, may seem to have achieved this, but they also radically simplify their lives so they have very little responsibility and rarely deal with anything new, so that there isn't much to be anxious about. On the other hand, maybe one reason they do this is to realize that even with all that simplification, they still feel the same human responses such as anxiety just as strongly, so they are more easily able to isolate those feelings in order to come to terms with them, and thereby be in a position to help others deal with them.)
So I think the next best thing to having a relaxation meta-skill might be the practice of acceptance, i.e. you make it a habit to consciously acknowledge when you feel anxious about certain things and to internalize the belief that it's OK to feel that way, if that's how you feel.