Prototyping a GUI with a customer

Always start with paper or paper-like mock-ups first. You do not want to fall into a trap of giving the impression of completeness when the back-end is completely hollow.

A polished prototype or pixel-perfect example puts too much emphasis on the design. With an obvious sketch, you have a better shot of discussing desired functionality and content rather than colors, photos, and other stylistic matters. There will be time for that discussion later in the project.

Jeff discusses paper prototyping in his Coding Horror article UI-First Software Development

Click the "Watch a video!" link at twitter.com to see an interesting take on the idea from Common Craft.

The "Napkin Look & Feel" for Java is really cool for prototyping. An actual, functioning, clickable app that looks like it was drawn on a napkin. Check out this screenshot:

Seriously, how cool is that?

I would suggest you sit down with your client and use a tool like Mockupscreens and develop the UI interactively. A benefit it has over Napkin LAF is that it does not require coding, or indeed development tools of any kind