Firstly to ensure any key human resources required to advise on WBS elements are in the room. I guess the only rules would be the same for those in meetings, things like everyone's opinion can be heard, no idea is a stupid idea (even if it is), no shouting and dominating the podium as it were etc. Ways to make it fun? Hmm I'm sure there are ways, but have only been in a war-room environment during a WBS session. Saving Changes...
Break up the group into smaller pods and have each pod decompose one or more branches of the WBS. Then have the other groups critiques them to identify gaps. Give prizes for most gaps identified AND for least gaps identified.
As a tutor of PMIEf program i did demonstrations to teachers on how to use the WBS so they can show to their kids. Teachers like it but they have "difficulties" on writing things down.
Some tips
Moderate, facilitate the session. Start by showing them but let them work.
Help to write down, register the activities on the paper, board (i prefer to use post it). Breakdown till work package.
Ask to turnoff the mobile or put it in silence mode.
It is an iterative and recurrent exercise. Saving Changes...
MANDA SATSHISHINGAManager| Ordem Dos Engenheiros de AngolaLuanda, Angola
Each team member have a voice and the ?? is welcome as soon as lead to an higher requirement or improve existing scope. Saving Changes...
Senior Projects Manager | Field & Marten AssociatesNew Westminster, British Columbia, Canada
Make it two sessions: Divide the team in sub-groups which each handling a certain work package. Next session, for the same sub-groups shuffle the packages so now you have two WBS for each work package.
Compare both, discuss with the whole team, reconcile and finalize the WBS all together. Saving Changes...