I just read a good article on using Usage Stories to help define requirements. Unlike use cases, which help define a user's actions with the application in specific situations, a Usage Story is a very high-level scenario. No if this, then this modeling, just one or two sentences on what the user is trying to accomplish and what, if any, top-level constraints are there.
At blinkx, we use Agile as our development methodology. Getting epics and user stories 'developer ready' is a challenge for us at times as so many of our product owners are also technical. And so are our business owners. We so easily devolve into technical requirements and constraints rather than forcing ourselves to stay high-level and focus on the actions and results, not the steps to get there.
I recommend the article and it can be found at: http://product-arts.com/joomla/articlelink/917-usage-scenarios-for-product-requirements