I'm a big fan of managing by objective. Wherever possible, I believe that PMs, engineers, testers and designers should begin their work by first agreeing to (or at least accepting) a list of user and business objectives that a feature or product should fulfill. This will be useful in framing the many discussions that will follow.
This week in Why Microsoft Had to Destroy Word Peter Merholz discusses how Microsoft made some tough decisions in the design of Word 2007. He specifically talks about how Microsoft and Tivo set out "design tenets" or "design mantras" which then governed the design and implementation of key features. Worth a read.