Do you know which are the common patterns for a graphical representation of a sitemap?
Hagan Rivers proposed a very useful model of representing a site's navigation during her presentation Escaping Navigation Hell, you should look into it. She says it's better to use an application map since the navigation system can be viewed as an application on itself that has the objective of taking you to the screen you need.
It's so much clearer than a regular sitemap, and it looks a lot better.
You'd try to represent it like a flow-chart with links and interactions between pages.
This is a sample of mobile phone menu navigation. This is just an idea.
to Clara Gaggero for more details
Flowcharts are the most common tool for visually representing site structure, as far as I know.
Here are a couple of examples: