Get a good roomSince in a Retro, you expect to make a significant change towards a meaningfully positive change, do not let the room hinder the team from coming up with good ideas for change.
FocusEvery minute spent on sidetracks and gimmicks destroys organizational value. Everyone has the responsibility to keep focused during the Retrospective and the Scrum Master's facilitation of the Retro should support this process as naturally as possible.
PreparationThe best way to get a meaningful result out of the Retrospective is by being prepared. On the other hand, being completely unprepared is the best way to make any meeting for any reason whatever - completely ineffective.
Both good teams and Scrum Masters prepare the Retro in advance. Then, they follow through and turn this well-prepared Retro into meaningful and highly effective action.
SummaryContinuous Improvement, is the only way to reach high performance. The Retrospective is the standard mechanism of establishing Continuous Improvement.
On the downside, poor Retrospectives will result in inherently low agility.
A Scrum Master must frequently reflect, "How can we make our Retrospective more effective?" - and adjust their own behaviour accordingly.