Here are some very basic XML "best practices"...
Favor the most strict document definition that is practical. Favor well-formed documents over no validation at all. Favor valid XML over well-formed XML.
Favor expressive validation over nominal validation. Favor schemas over DTDs. However, if you toolkit makes automatic validation of DTD-based documents easier than schema-based, this may make you favor DTDs.
Favor automated testing... but test no matter what. Ant has an