Web mockups

The marketing gurus at my work want to redesign our online ordering system. What we ended up with is certainly problematic, but at the time was about as much as we could get into the scope of this huge project. It makes sense, now that the dust has settled, to revisit and massively improve it.

The trouble is, there were a lot of thought that went into the specific flow we finally got. A lot of complex, important decisions were made, and they haven't necessarily been documented.

To help in our design process, I suggested we go straight to proper mockups, skipping the wireframe stage since we're pretty clear on what we want. Now I'm digging around for the best way to do this.

The mockups need to have the majority of the form logic we'll be using, as the form logic is the most important part of our improvements. We need to shake out the dependent questions we'll need to ask, so we can structure it all properly.

So, dear lazyweb, does anyone have any tips for tools or techniques to use in mockups? These need to be more than just "white site" type stuff, though styling isn't anything like as important as the underlying logic.