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.
Let team members appreciate each other and end positively Source:
Agile Retrospectives who took it from 'The Satir Model: Family Therapy and Beyond'
Start by giving a sincere appreciation of one of the participants. It can be anything they contributed: help to the team or you, a solved problem, ...Then invite others and wait for someone to work up the nerve. Close, when no one has talked for a minute.