This article pretty much sums up exactly the process I go through when wireframing a new web app. Personally, I use adobe fireworks to do this, and I've gotten fast enough with it that when I'm in the zone, I can express myself in it almost faster than writing it down or talking.

The most important point here, IMHO, is that wireframing should be done in a medium that you can change fast without much effort or fear.

This is why I don't like the whole HTML/CSS wireframing technique. That works great if you only have 1 or 2 directions to go in, but what if you're exploring 7? You end up having to manage 7 different html and css files. In fireworks, I just click "duplicate page" and nudge a few UI elements around.