Designing, building, and launching a new product or service is both complex and exhilarating.  I believe one of the keys to doing this successfully is to write a narrative about how your target user uses your product/service in the context of a day in their life.  I can't claim credit for this approach as I learned it from the guy who gave me my first shot as a product manager (thanks again Jim). 

This sounds easier than it is but thinking through what your target does before, during, and after using your product/service forces essential features to the forefront, surfaces substitutes, flushes out bad assumptions, and adds a persona to your internal engineering and marketing meetings.  Instead of "the user does this or that" you discuss how "Jim does this or that."  This also further crystallizes thinking on target markets and go-to-market approaches.

If you can't tell the product/service story through the eyes of a target persona, you're not as focused as you need to be.  Start with what they do first thing in the morning and describe everything they do in a given day until they go to bed working your product/service into the story.  Give it a try.  I promise it will be a revealing exercise.