Since I upgraded to 6.3.2, I use XML Tools.
install XML Tools via the Plugin Admin (Plugins → Plugins Admin… Then search for “XML Tools”, check its box and click the “Install” button).
use the shortcut Ctrl+Alt+Shift+B (or
menu → Plugins → XML Tools → Pretty Print)
In older versions:
menu → TextFX → HTML Tidy → Tidy: Reindent XML.
Install Tidy2 plugin. I have Notepad++ v6.2.2, and Tidy2 works fine so far.