User stories are used to describe the functionality delivered in a product or system. Two common challenges we see recurring, even with experienced teams, are (1) they spend far too much time planning and estimating, getting trapped in the details, and (2) they have difficulty finishing user stories in a time-boxed iteration or Sprint – often realizing, despite the promise of further conversation, that they just didn’t know enough about the user story before taking it on. This hands-on session introduces participants to two techniques to deal with these problems. The first is Silent Grouping, which allows teams to collaboratively size even very large backlogs in minutes. The second is Definition of Ready, which is a set of agreements that lets everyone know when a user story is ready to begin. Combined, these techniques can save your teams literally hundreds of hours in time.
http://submit2012.agilealliance.org/files/session_pdfs/Agile 2012 Working Effectively With User Stories.pdf