How I keep a grammar notebook
A small habit that made my own English noticeably better.
I write down every mistake I notice, mine or a student's, together with the corrected sentence and a short reason. Once a week I read the page aloud and cross out what no longer trips me up.
The notebook has three columns: the wrong sentence, the right sentence and the rule in one line. Nothing fancy, but after a few months the repeated errors became obvious and the page became shorter.