People sometimes talk about "relaxing" like it's a general skill. Either they say, "I'm bad at relaxing," or something like "I need to learn how to take things less seriously," etc. But I think we can't help but be more at ease with familiar things and more anxious about new things. So learning how to be calmer about some things (by making them familiar) doesn't lead to an ability to automatically feel calmer about new things. For example, I noticed I have been feeling way less stress about starting at Google than I did about starting at Microsoft 5 years ago, and moving to a new state and taking care of all the associated tasks has been a piece of cake this time. But none of this means that I have learned how to take arbitrary things in stride. I still notice myself worrying about plenty of issues that I know will probably not turn out to be a big deal.
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.
