Well, there are many design philosophies, and best practices and recommendations for XML schema design exist (utilize your favorite search engine for that). Another way of approaching the design of new XML schemas is to look into how existing XML schemas have been designed (similar to learning from existing code libraries). To get you started, here are some resources:
- Susan Malaika's "Get started with Industry Formats and Services with pureXML" has a nice overview of standards and provides links to them.
- Wikipedia has a list of XML schemas.
- If this is still not enough, Altova also has a Schema Library.
- Finally, in Paul Kiel's article "Profiling XML Schema" is an overview of some industry schemas and what XML Schema features they use and which they avoid. This can give you an indication on what to watch for when you start with your own XML schemas.