To compliment Jan Doggen's answer, here are a few tasks you have to do continuously.
Demonstrate the values
As a leader in the community, you and other leaders, need to demonstrate the professional and polite atmosphere you want. Engage with users in the manner you expect others to follow. When you disagree with other users - and you will - do so in a respectful way. The goal is to come away from the disagreement with all sides still looking professional. Arguments can be heated and opinions and emotions can be strong, but your task is to show that such arguments can be handled in a professional manner.
Enforce the values
If you see a user toeing the line of acceptable, call them on it. Explain what it looks like it happening in a clear manner and encourage them to take a step back. The user may (or may not) realize what they are doing and the opinion of another can draw the behavior to their attention. With this, though, comes moderation. If the user doesn't follow your advice it may be time for a temporary suspension (or other course of action your community and leadership have decided upon).
Encourage bygones to be bygones
At some point, you're going to upset another user. This will be either because you disagreed with their position or because you had to perform a moderation action against them. When the situation has been resolved (argument is over, suspension is complete, etc), don't bring it up publicly at a later date. It happened but you've moved on. By showing your community that you can keep moving forward and still respect others for their opinions or can allow a suspended user to return, you are showing others should be able to do the same.
Ask the community for feedback (and act on it...sometimes)
The community will grow and others will have ideas on how to make it better. Give them a way to provide that feedback or actively seek it out. Encourage community discussions about changes the leadership team is talking about. When you implement a new idea, thank the community for their feedback. If you don't implement an idea, don't just leave the community hanging. Explain why you didn't implement a suggestion. You don't need (or want) to follow everything the community suggests, but by explaining why you aren't, you show that you've considered the idea.