Following Luke Wroblewski’s 2009 Parti & The Design Sandwich talk from Interaction09 in Vancouver, he has written a summary on how to go about developing Design Principles.
In order to be most effective, however, design principles need to provide teams with a way to gauge design decisions. That is, they should be specific enough to help groups of people choose between different design options.
Unfortunately, many team’s first tendency when creating design principles is to go too broad. Principles like “make it easy to use”, “keep it fast”, or “put the user first” are usually some of the most common ideas that spring to mind.Luke Wroblewski
Update: Dave Malouf has recently referenced this talk What are and how I have come to use design principles in my practice