It helps me to (attempt to) explain my mathematical thoughts to another person. It somehow helps even if the conversation itself isn't fruitful: the act of putting math explicitly into words can be revelatory. I think that's partially why new ideas often pop into my head right as I'm walking away from a meeting; the simple act of walking is probably also responsible. (Better late than never!) Even if another person isn't available, just trying to write things out formally (in good English) can have a similar effect.