I've spent rather too long at times (years) "teaching" and not advancing myself. More recently I've observed more carefully how some of my (Japanese) teachers push and train themselves while teaching and that is what I seek to emulate and challenge myself with.
Equally, the good classes are where I start with an idea, and then find that the interaction with the students present and my reacting to issues they are having leads me to new insights during the class, and better ways to explain things and teach them. Time flies by and we all have fun. This doesn't happen every class
A great class gives the temptation to repeat it next time (usually students are slightly different) - and it never works well second time around - lacks the spontaneity and creativity.
I like the idea expressed by Richard Moon (6th dan and 40+ year student of Robert Nadeau) when one of his students said "But you are just learning out loud!" It has become one of my mantras...