Set the ground rules at the onset:
Constructive critisism only.
Remember that some people have put a lot of effort into the show, so make comments, but be kind.
Discussion is encouraged, but it has to be orderly. (Establish this from the onset, for example, if there is a comment directed at a department, the department receiving the comment has first response, the director next, etc etc etc, then open the floor)
I love doing post mortems, it helps me to further my craft, and it helps others to do so too.
(A few to add - I had to leave earlier)
Conference table setup (ie not lecture theatre or anything like that)
Make sure there is tea and coffee and maybe something to eat on the table
Start with reviews of your own performance, problems, solutions etc and what you would do better next time etc, then move around. This sets the tone... don't get into 'suggestions and constructive critisism" until after everyone has offloaded their "Problems they have already noted"