I'm of the Stop Thought school. Part of my problem is that I can become obsessive and morose and self-pitying. Since ruminating and dwelling on negative things or things I need to resolve within really doesn't solve anything or change the reality (such as with a breakup), I actually DO say "Stop it" out loud. I don't like to torture myself with my own thoughts.
Another technique I've used when something just will not stop popping up in my head is to have a Worrying Time every day. I spend ten minutes - on a timer - journaling about all the thoughts about that thing. Any time during the day that it still pops up, I say to myself, "Okay, think about that in the Worrying Time." Ten minutes a day is enough for me to type out all the venom. I sort of naturally tend to bend toward solutions because I don't like feeling bad, so after a few sentences of negative spewing, I start seeing the solutions and type that out, too. I think we instinctively want to heal. I've found my journal and Worrying Time to be really helpful when something absolutely needs resolution.