i have a practice where when my mind starts doing it's thing i simply breath in, hold my breath for a very short time (it only takes a moment or two to short circuit the rant.) then breath out and hold breath for a moment or two, and continue to repeat.  in the beginning of using this 'tool' the mind might return to it's rant right where it left off between breath holds.  yet over time i have found that doing this as an ongoing practice, when necessary, has trained my mind to stop almost at the point of THINKING ABOUT holding breath, etc. and a ceased rant these days rarely returns.