Erik Petersen posted a note on his blog that referenced a website for, of all things, a bicycle touring group that needs to run effective meetings.
Lori Walter, writer of the "Guides: Meeting Facilitation", describes a technique for managing the flow of conversation when facilitating a session where a group/team needs to think together. I can imagine it working well in a retrospective where the need for consensus on action is high as well as the contention about which direction to take the team. She calls it the Levi Hand Signal Technique.
Walter says, "LHST allows meeting participants to register their intent to make two distinct kinds of comments: those that are directly in response to someone else's comment ('reactive comments') and those that are separate thoughts ('unique comments')." And she goes on to describe a "twinkling system": ..." a way for people to signal their agreement with what is being said without having to all put up their hands" which reminds me of the patterns community system of agreement, "GUSH!"
Check it out!