The way to live in the present is to remember that "This too shall pass." When you experience joy, remembering that "This too shall pass" helps you savor the here and now. When you experience pain and sorrow, remembering that "This too shall pass" reminds you that grief, like joy, is only temporary.
Dorothy tries to sum it all up before leaving Oz. "It's that if ever I go looking for my heart's desire again, I won't look any further than my own backyard," she tells Glinda, the Good Witch of the North. "Because if it isn't there, I never really lost it to begin with. Is that it?" "That's all it is," confirms Glinda.