Better User Stories - online video content from Mike Cohn is currently available for free
Many of us in the Agile community know Mike Cohn as being one of the leading experts in Scrum, Agile, and especially User Stories. He has a video course, typically as paid-for subscription. On occasion (like now!), he makes some of this material available for free.
Since getting free tips, tricks and techniques from the expert is never a bad thing, here's the link to Better User Stories.
Several of us in the Agile community here at Pega have been involved in leading trainings on Scrum practices. During these sessions, we've discussed story prioritization and release planning. User Story Mapping, which Cohn discusses in the first lesson, is a fantastic way to break down story work. Highlights include aligning upcoming effort with short-term objectives, defining the participants of a Story Mapping exercise, and walking through the mapping itself.
While I'm not deeply involved in building out cases in Pega, I believe that the structure of User Story Mapping aligns strongly with how cases are defined. Cohn gets into the modeling of this around the 13:30 mark of the video.