XML-centric thinking means a focus on what should be included on a web page, not how it should be presented. This is contrary to the method used by many web designers, who tend to visualize the resulting web page first.
One method that may help the XML-centric process (especially for designers who are trained to be visual) is keep asking questions about how a web page may be presented in different means: text-only, HTML and audio (screen reader). The common parts of these different presentations are a good beginning point.
Alternatively, one can also keep in mind that it is the “what”, not the “how”, that is important in an XML-centric process.
It may be helpful to write sample XML documents as examples of what kind of information should be captured, and how that information is structured. This can be done (as examples) even without a proper DTD to refer to.