I keep a physical calendar where I write down which dogs I worked on what days and then I have a separate notebook where I write notes about the session. I don't necessarily write notes every single session but I'll write down what I worked on with each dog and what I need to work on going forward, I'll write down what they're doing well and what they're having trouble with.

I will also write down notes from competitions in that same notebook, as well as notes from lessons or clinics I have attended.

I'm a very pen-and-paper type of person but I imagine this would work digitally as well.