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.
Condense many possible actions down to just two the team will try Source:
Lydia Grawunder & Sebastian Nachtigall
Hand out index cards and markers. Tell everyone to write down the two actions they want to try next iteration - as concretely as possible (SMART). Then everyone pairs up with their neighbor and both together must merge their actions into a single list with two actions. The pairs form groups of 4. Then 8. Now collect every group's two action items and have a vote on the final two.