It's a bit more analog, but one piece of advice I got in a teaching seminar was to only ever write true things on the board. This is important because students often zone out at just the wrong time and when they come back and realize they missed a bit of class, they'll glance at the board and furiously start copying down whatever you wrote without knowing the context. If it turns out what you wrote wasn't true, well now a whole bunch of students copied down the wrong thing and are going to be extremely confused why the example they copied verbatim from class isn't working.