Once you begin using XML you will invariably be drawn into accessing the contents of your XML format files by using XPath.
Please, don't manually walk the Document Object Model ising "parent" and "sibling" operations or any other sort of nonsense! You'll only be creating code that is very hard to debug, manage, modify and maintain!
Moreover, XPath is the core of many XML applications, espeicially XSLT. XSLT will surely be your next stop along the XML technologies path and it relies heavily on XPath. What does that mean? You should become fluent in XPath!
Here is a Flash version of the tutorial as well as the corresponding DHTML version of the same.