How do you run a sprint retrospective to maximize participation and honesty? And what about follow up on action items? [closed]

Question

I've seen it done as a free-for-all where the scrum master just asks everyone to throw out comments. Or a time-boxed session where "anonymous" comments are posted on the walls bucketed by category. My current team is constrained by being spread out, so it has be done online/over the phone. What is everyone's best experience with soliciting constructive feedback? And how do you follow through on action items for improvement?