Known as the "Sticky Shotgun" approach. The tactical steps are:
- Give everyone a pad of stickies and marker.
- Take 5 minutes to write down whatever comes to mind from the release.
- Stick each sticky in correct "zone" on whiteboard. (Keep Doing. Less Of. More Of. Stop Doing. Start Doing.)
- Give a very quick explanation while adding it.
- Group the stickies in each zone into into affinity groups.
- Discuss each affinity group and try to identify what needs to be done to solve the issues; or what was done that makes them worth keeping.
- Vote on 2-3 most important issues so the team can prioritize/know what needs to be addressed first.
That's probably a good style to use given your goals, but you might also want to consult this list of retrospective plans to see if there are others that would fit your needs. http://retrospectivewiki.org/index.php?title=Retrospective_Plans
One thing to do, if you can, is have a 3rd party facilitate the retrospective - that person can help keep the discussion on track, cut off tangents, move everything forward, and make sure the space is safe for everyone involved. If you can't get a 3rd party you just need to make sure that the person facilitating knows their role -- they are not to participate, but facilitate.