Proper database modeling is more than just knowing how to draw diagrams or about normalization rules.  A huge portion is understanding the subject area, data flows, processes and goals.  I would also include the ability to look ahead at possibilities and to judge what is feasible for both the near term and future capabilities.  Besides Entity-Relationship (E-R) diagrams, I have also witnessed the decline in the use of Data Flow Diagrams (DFD).  In my experience, DFDs have been very instrumental not only to document data flows within a system, but more importantly as a means of discussion to ensure that I correctly understand the system properly or as a tool to help others to understand.

I have often wished I could distill my 30+ years of experience in data modeling into a book, but I have been plagued with not only a lack of time but also the inability to turn my experience and knowledge into a set of discreet rules or a methodology.  Perhaps it's because I unconsciously follow a set of rules that I am not aware of and need to identify them first! 😛