I don't even remember where I learned it, I think it's some meditation thing? I'll specifically think, "I wonder what my next thought will be." And then try to 1.) keep it quiet as long as possible and/or 2.) if I fail, just try to pay attention to what the thoughts were. Simple enough idea that it stuck with me, you can try it at your desk, and it's kind of a fun little challenge. But also kind of impossible.
This is great! There seem to be quite some tricks out there, one I learned is to focus on the temperature of your breath and where exactly you feel it while breathing. In the beginning it's just little moments of thoughtlessness, but you can make them last longer with practice.
There's also this reply I read on /x/ like ten years ago that has stuck with me, to the question someone asked, "how to meditate" – "Notice the space between 2 thoughts? Enter it."
God I wish I could quiet my brain for just a minute. I always have thoughts going in and out like crazy. It's hard to keep shit straight. I go back through the various notes I wrote for myself every day to try to organize the random shit I think about and most of it is absolute nonsense.