I do not recall if I have mentioned this before, but I find that it’s helpful to tell the students that you are their “guest musician” rather than their “sub”. I even write my name on the board with the title of guest musician beside it. I also attempt to show my musicianship at the beginning of the class in some way, usually through performing on an instrument. This does three things: shows that you are truly a musician, exposes students to a professional playing an instrument, and will eventually give you a good reputation when you come back to those classes. I often am asked by students who know me, “Will you play Beethoven on the piano?” (The mean Fur Elise — they love that one).
And also remember, “The don’t care how much you know until they know how much you care.”