In the post yesterday, I started to cover some analytic ways to think about building up data intensive diagrams. In future posts, I'll spend a little more time talking about how to use those tools for specific diagram types.
Of course, another great way to build great diagrams is to learn from examples. For network diagrams, there is a great site out there that lets you take a look at some really amazing diagrams (as well as some impressively horrible ones): http://www.ratemynetworkdiagram.com/.