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.
Gauge participants' satisfaction with the retro on a scale from 1 to 5 in minimum time Source: ALE 2011, Corinna Baldauf
Put sticky notes on the door with the numbers 1 through 5 on them. 1 is the topmost and best, 5 the lowest and worst.When ending the retrospective, ask your participants to put a sticky to the number they feel reflects the session. The sticky can be empty or have a comment or suggestion on it.