We have noticed on our current project that during retrospectives people usually raise good and bad things from the last days only. At the moment we have a 3 weeks iteration and it's hard to remember what happened on the first days. I have already used from 1 to 4 weeks iteration and each project has its own reasons for choosing this length. To help the team improve around things that happened during the whole iteration, we have a box called the Future Retro Box, where people can all keep adding notes throughout the iteration.

The reason why the box is called Future is because it will be opened during the next retrospective. The items inside the box will bootstrap the retro wall when people can add other ones.

Make sure the box is in a visible point of the work environment.

Another option would be to have an on going retrospective wall, as suggested by my friend Lachlan. However, this might work better for a co-located team, we are distributed. We tried using a wiki for that… It didn’t work… No one adds comments to the wiki… Why? Perhaps the wiki is a Refrigerator as opposed to a Radiator. The box has been working fine so far. The notes are not visible, but the box itself is.