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.
Introduce SMART goals (specific, measurable, attainable, relevant, timely) and examples for SMART vs not so smart goals, e.g.'We'll study stories before pulling them by talking about them with the product owner each Wednesday at 9am' vs. 'We'll get to know the stories before they are in our sprint backlog'. Form groups around the issues the team wants to work on. Each group identifies 1-5 concrete steps to reach the goal. Let each group present their results. All participants should agree on the 'SMART-ness' of the goals. Refine and confirm.
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.