Liquid XML Studio is pretty good at real-time interpretation of your XPATH queries.
Cooktop also lets me run my XPATH queries and shows me the XML and HTML generated by running the XSLT against a given XML.
Also, a colleague tells me that newer versions of Adobe Dreamweaver allows you to associate an XML file with an XSLT file and run the transformation.
Whenever possible i use Subversion for change-tracking.
And for navigation, i most often use VIM (or VIEmu) and sometimes a custom Visual Studio extension that builds an index of the current document's xsl:template and xsl:variable nodes to provide one-click navigation to the root entries.
