Nothing. If you don't give energy to whatever is passing through your head, maybe it will do just that, ie pass.
Tell your mind to hush. Calling out the monkey mind modus operandi may quell the chatter. I have a friend who can release her racing, often worrisome thoughts by saying "stop."
Work out. Consuming oneself with the physical, if it involves sufficient rigor, eventually consumes and exhausts the mind. This is why I do Bikram Yoga, but jogging or speed walking work wonders, too.
Prayer. Some issues are way too big for me. I turn those over to God. And you can, too, even if you're not religious.
Chant. I learned about chanting several years ago when I read "Healing Mantras" by Thomas Ashley-Farrand. I have found repetition of this mantra to be akin to an affirmation (even though I don't pray to a Hindu god as I chant).
Om Gum Ganapataye Namaha
which loosely translated means "remove obstacles from my path."
Most of my unsettled, restless mind involves life's obstacles, so creating a mental picture of myself moving through life and removing obstacles in my path is the perfect visualization for me. I say this mantra affirmation in my mind, but the sound of your voice adds value as it carries energy.
The recording below is a beautiful, melodic version of this mantra. If you are feeling like you need some mental rest, hit PLAY and sit quietly and let the words travel through your monkey mind for the next seven minutes or so.
Vishuddha SoundTouch - OM GUM GANAPATAYE NAMAHA