I think that one of the key things to make retrospectives valuable is to use suitable exercises. By valuable I mean exercises that help teams to better understand what has happened and to take action. Suitable will often depend on the team and situation at hand.
The retrospective facilitator (often the scrum master) should have a toolbox of retrospective techniques, and be able to pick the most effective one. Some of the techniques to do retrospectives are asking questions, state your feelings with 1 word, 5 times why (Root Causes) or asking why, solution focused/strengths and retrospective of retrospectives.
@BenLinders
Co-author Getting Value out of Agile Retrospectives
The retrospective facilitator (often the scrum master) should have a toolbox of retrospective techniques, and be able to pick the most effective one. Some of the techniques to do retrospectives are asking questions, state your feelings with 1 word, 5 times why (Root Causes) or asking why, solution focused/strengths and retrospective of retrospectives.
@BenLinders Co-author Getting Value out of Agile Retrospectives