Perhaps you could ask a friend to tactfully observe the two of you together and see what she thinks. I'd suspect that body language isn't something you can learn to "do better," but rather, a subconscious indicator of his feelings.
My DH gets after me all the time for mine - I tend to cross my arms, which is sort of a "closed off" sign. He says I appear unfriendly that way. And he's right, I think. The issue is that he has a problem with my unsocial behavior, and I don't.
