When you hear teachers say, “Watch your thoughts,” it is an actual sadhana. (spiritual discipline.) It is very potent when applied, but I have seen it used in ways that somehow create more thought about the thought one is watching, which just reveals how potent and effective this practice is, that immediately the mind wants to make it interesting, personal or get something out of it. So here is the practice, if you would like to engage:
However you are, wherever you are, sitting down for meditation or out and about this practice can be initiated. Note: It is not so you can decide which thoughts are good or bad, or even expansive and contractive, although there can be some value in recognizing the difference. This is not part of the watching.