Continually look for new ways to keep the team focused on quality.
Do what you can to make sure the workspace is conducive to successful XP teamwork.
You can't have too many whiteboards.
Celebrate small, and big, successes.
Use a timer if standup meetings run too long.
Use a project wiki to help the team communicate and transfer knowledge.
Gather only metrics that provide useful feedback for the team and the organization. Keep them simple.
Your project needs separate development, test, and production environments, all configured the same way and containing the same software components.
Encourage smart use of good source-code control tools.
Snacks and toys make every project go better.