Mike Fitzgerald explains how to use SGML to help auto-tag XML. His column is actually an extract from XML Hacks, a book incorporating many of Sean McGrath's shortcuts.

XML Hacks is a roll-up-your-sleeves guide that distills years of ingenious XML hacking into a complete set of practical tips, tricks, and tools for the web developers, system administrators, and programmers who want to go far beyond basic tutorials to leverage the untapped power of XML. If you've ever tried to go to a web page automatically and then parse the information by examining the string you got back, you can appreciate XML. While not perfect XML is probably the most practical option for packaging data that can be read by both humans and computers.

The book is very useful if you already deal with XML but it's certainly not a primer. You need to understand DTDs, schemas, and SOAP before you can really use every XML hack demonstrated.