"Never look at the trombones. You'll only encourage them"

This quote by Richard Strauss is most likely suggested to be funny or sarcastic. The statement "Never take a look at the trombones" suggests that the trombone gamers are looking for focus, and that by considering them, you will be motivating them to continue looking for that attention. It is feasible that Strauss was making a playful stab at trombone players or musicians that he felt were overly assertive or attention-seeking.