I do a lot of DocBook XML editing, either at my job or at home. Because of that I’ve built up a pretty customized .emacs file. Every so often I meet another person whose also found themselves having to edit a bunch of XML. The most fantastic thing about nXML mode I think is the automatic slash completion feature. It works like this: If I have an open element, say I’ve started an <xref>, you can configure nXML mode such that upon typing the closing </ characters it will complete that sequence for you. I can just never remember how to set that option in emacs. So today I’m taking the time to finally document that procedure.
For even more fun, use the C-c C-f macro which will auto complete your current block, regardless of your position inside of it. For additional references, I invite you to check out the docs NM Tech has posted on nXML-Mode.