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.
What would make the next iteration a perfect 10 out of 10? Source:
Prepare a flip chart with 2 columns, a slim one for 'Rating' and a wide one for 'Actions'. Everyone rates the last iteration on a scale from 1 to 10. Then they have to suggest what action(s) would make the next iteration a perfect 10.
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.