Leading by example is the best way, in my opinion. Even though they may not follow it or get the hint, the better you are, the better they can try to be.
For example, I had a case where a superior was very unprofessional and used profane language while conducting business. As the person let out all their frustrations, via a conference call, my direct report and I simply responded in a calm and professional tone and immediately this person calmed down. Soon after, my direct report followed up with a very firm, yet assertive, email stating that their conduct was "unprofessional and inappropriate" and that our department is only the middle person in the situation and providing the best service for his team as allowed, but that we understood their frustrations as well and will be taking them into account. Nonetheless, the subsequent emails and interaction have been quite cordial ever since. Although the person never officially apologized for their behavior, they did acknowledge their error in judgment.
As a result, we kept our cool and professionalism while teaching, or better said, reinforcing the NEED for workplace etiquette no matter how difficult the circumstances.
Great question and very near and dear to my heart.