Guy Bueachamp of Business Analyst Solutions has written a short article called The Fundamentals of Business Analysis and posted it at Modern Analyst. In it he proposes a structured framework to ensure requirements are valid, based upon the relationship between the projects objective, planned benefits and scope.
I have copied one of the diagrams from the article to get you interested. Go take a look.
It also has an interesting comparison between the roles of business analysts and architects (from the construction industry.)