Practice in front of the entire team (it's hard to keep a straight face in front of your peers sometimes), and then practice in front of some adults you've never met before. Remember that the judges may not be that familiar with FIRST so avoid saying things that only "insiders" would understand.
NEMO has published a white paper,
Judging Hints at the Regionals, which talks about interacting with the judges outside of the Chairman's interview that might be useful to review with your team.