There are several very good resources for schema design. The list below shows a few I recommend.
"ASC X12 Reference Model for XML Design," Type 2 Technical Report, by the ANSI Accredited Standards Committee X12. Available online at www.x12.org. While focusing primarily on a semantic architecture for X12's work on XML, this report discusses several syntax-level issues and recommendations regarding document and schema design.
"UBL Naming and Design Rules," by the Naming and Design Rules subcommittee of OASIS's Universal Business Language Technical Committee. Available online at http://oasis- open .org/ committees /ubl/ndrsc/#documents. This is a very good framework for document and schema design. The specification itself generally just defines the approach and doesn't discuss the advantages and disadvantages of various approaches. However, various working and position papers on the Web site may discuss background, issues, and alternative approaches.
"XML Schemas: Best Practices," developed collaboratively by The MITRE Corporation and members of the xml-dev list group , hosted at Roger Costello's xFront site. Available online at www.xfront.com/BestPracticesHomepage.html. You can't go wrong starting here.
"XML Technical Specification for Higher Education," by The XML Forum for Education of the Postsecondary Electronic Standards Council. Available online at www.pescxml.org/. This document offers the perspective of a group in one particular industry sector regarding document and schema design practices. It includes discussion of tradeoffs and several examples.