I came across this link (http://social.technet.microsoft.com/wiki/contents/articles/19692.biztalk-server-deep-dive-in-schema-design.aspx) which does a nice job of giving a deeper overview of XML schema capability and design.
Although this comes across as BizTalk-centric, my thoughts are that the overall information is W3C compliant. This means that Microsoft, in their own way (in this case, using BizTalk as the schema editor) which does try to conform to XML standards. This is a good thing, of course. Any tool could be used, for that matter, such as Altova's XMLSpy, to develop XML schemas (.xsd file).
I've seen many developers load in an XML document and immediately using xPath to get information. Depending on the scenario/environment, this may lead to issues that are difficult to debug, especially when working with complex (and sometimes slightly different) XML documents. It's much better to bring in and XML document and use it as a 'type' via a schema, rather than treating it similar to a text file with 'some extra functionality'.
Also, another site (http://www.xfront.com/BestPracticesHomepage.html) which discusses some best practices...
