The say you fail to plan, you plan to fail. Having a good Sitemap BEFORE you start helps create a better picture of how everything links together, and helps you discover navigation issues early.

Thankfully, there are some really great online tools to help you map your website, and collaborate with others over the web.

This page has a great wrapup of some of the mindmapping tools available: 9 great ways to make mind maps and flow charts online