Prototyping - in a nutshell, it's testing new developments out before you either build the website or change an existing one. Saves a lot of pain in the long run. And probably a few heated discussions too.
For those new to the idea of prototyping:
Introductory article on paper prototyping from www.alistapart.com
Prototyping with MS Visio is pretty quick and easy. Particularly with this tutorial and free templates:
Visio templates for paper prototyping, plus supporting article, from www.guuui.com