Each time I venture into JIRA
plugin add-on development, I scratch around for a moment trying to remember just how JIRA page decoration works. It doesn't seem to be well documented anywhere easily accessible.
I normally just go back to the previous JIRA add-on I was working on and look in the atlassian-plugin.xml for web-sections/web-items and some velocity files for meta tags. A liberal sprinkling of these generally produces the page layout that's required for a particular job.
I was going to document this with screenshot and code samples, but happily stumbled upon Jobin Kuruvilla's blogpost Use of decorators on JIRA pages.
This post is now slightly out of date because of the new page layouts from JIRA 6, but the essentials hold true. It's an excellent tutorial. Nice work Jobin.