Retrospectives - too important to leave them to amateurs

What are your experiences with retrospectives? Boring, little to no effects, issues are not addressed,...? I saw too many teams doing bad retros or even stop doing retros. Without good and effective retros there will not be improvement. To facilitate retrospectives that inspire participants and let them talk about issues openly requires a well skilled facilitator. I will give insight of what I learned from more than 300 retrospective. You will get an overview what a well skilled facilitator needs to know and to do.