Sorry I didn't answer sooner . . . started a couple of times and got interrupted!!!
I was getting quite successful at this a couple of years ago . . . when I was spending more time on spiritual pursuits (a prayer/meditation group, reading, individual meditation). Somehow it just started to come more naturally. In the meantime, I have "slipped" somewhat and it is taking a bit more effort. I used to try to "force" my thoughts and was rather hard on myself for transgressions. That doesn't lead anywhere productive. Just being aware of the unproductive thoughts is helpful . . . then replacing them with something more positive.