Set up a 'retrospective mailbox' at the beginning of the iteration. Whenever something significant happens or someone has an idea for improvement, they write it down and 'post' it. (Alternatively the 'mailbox' can be a visible place. This can spark discussion during the iteration.) Go through the notes and discuss them. A mailbox is great for long iterations and forgetful teams.
Get a sense of everyone's position and reach consensus Source:
When the team can't decide between two options, create a big scale (i.e. a long line) on the floor with masking tape. Mark one end as option A) and the other as option B). Team members position themselves on the scale according to their preference for either option. Now tweak the options until one option has a clear majority.
Listen to others talk behind your back - and only the good stuff! Source:
Form groups of 3. Each group arranges their chairs so that 2 chairs face each other and the third one has its back turned, like this: >^<. The two people in the chairs that face each other talk about the third person for 2 minutes. They may only say positive things and nothing that was said may be reduced in meaning by anything said afterwards. Hold 3 rounds so that everyone sits in the shower seat once.