How to best prepare the mind for a problem identification brainstorm?
I'm wondering if your approach to "identify[ing] problems, not solutions," will include so-called reverse or negative brainstorming, in which participants are directed to consider "what can be done to break something instead of focusing solely on narrow-minded fixes,"1 to be "highly critical, rather than highly creative."2 If this is the case, the group could "warm up" by applying this technique to some other area: their own lives, their favourite sports teams, public policy issues, etc.

1 Stephen C. Harper. How to Solve Problems with Reverse Brainstorming, posted on his TalentSpace Blog, December 14, 2011.

2 Harper. Extraordinary Entrepreneurship: The Professional's Guide to Starting an Exceptional Enterprise (Google eBook), 2005, p.78.