I like and use all of the great ideas posted above ! !
Additionally, when my mind is sufficiently clear to do so, I like to make a game of thinking of 3 different ways to reframe whatever is troubling me. This exercise causes me to realize that there is more that one way to experience something. I like to make at least one of the reframings into something ridiculously funny. It can help to lighten my spirits and not take myself quite so seriously.
I sometimes find that I actually like one of the reframings better; adopting it greatly lessens recurrence. I usually find that one view of things is as good as another. In the immortal words of Lily Tomlin: "What is reality except a collective hunch?"
But without regard to whether I choose to adopt an alternative framing, the exercise is enough of a distraction to help break the mental ruminations.