STOP is the short instruction as well as the long one.
Pema Chodron describes STOP as sit, stay , heal.
She suggests that as soon as you become aware that you are lost in the detour you could try to stop, sit still and for a moment or so do nothing. Do not try to change the thoughts and feelings nor react to them, but stay with them and allow them to be felt.
Although this may be uncomfortable and you want to give in to your usual habitual response she asks you to wait, to sit , stay and then heal. By waiting and allowing the thoughts and sensations to be heard and felt more fully they allow the back door to open and the thoughts to finally flow out along with a return to more happiness.
Living A Happier Life: Ideas, Inspiration & Practices 