I find the old Boy Scout's motto of "Be Prepared" fitting for this discussion. I like to give examples for every topic of discussion, such as, "we do it this way because" or "this is what happens when". My activities, attitude and conduct are also examples of how a professional looks and acts. Without realizing it, we are a continuous example of that. Good class preperation shows our students what preperation and professionalism looks like, even if it is not a direct part of the lesson. We show them what is acceptable. You do your best and hopefully, they will too.