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.
Collect what to start, stop, continue, do more and less of Source:
Draw 5 spokes on a flip chart paper, dividing it into 5 segments. Label them 'Start', 'Stop', 'Continue', 'Do More' and 'Do less'. Participants write their proposals on sticky notes and put them in the appropriate segment. After clustering stickies that capture the same idea, dot vote on which suggestions to try.
Divide a flip chart in 3 sections titled 'More of', 'Same of', and 'Less of'. Ask participants to nudge your behaviour into the right direction: Write stickies with what you should do more, less and what is exactly right. Read out and briefly discuss the stickies section-wise.