If you cannot stop worrying over something in the past or what might happen in
the future, shift your focus to the inhalation and exhalation of your breath.
Or recite this mantra: om mani padme hum (pronounced "om mani padmay hum").
Since the mind cannot concentrate on two things simultaneously, either of
these meditations causes the former worry to fade.(p.133)
-- His Holiness the Dalai Lama, "How to Practice: The Way to a Meaningful
Life", translated and edited by Jeffrey Hopkins