Building UI's is always a difficult thing. It gets even more complicated when the clients don't have a clear vision of what they want. At my workplace, a UI definition is usually given as 'make it look like the other ones'. Of course, everyone is always shocked and amazed down the road when the different UI elements don't seem to mesh and there is no common look and feel.
I've read enough articles on paper prototyping to be sold on the concept. Essentially paper prototyping encourages designers/developers to draw out sample UI's on paper rather than try to create them in some tool like VB or FrontPage. The end result is something that is easily changed depending on the feedback.
Paper prototyping also helps to avoid problems that occur when clients (or even some PM's or managers) see what they think is an almost-finished UI. To them, the user interface IS the product. All those business objects and data abstraction techniques are just developers ways to make themselves seem smart, right'
I found a few articles on how to make paper prototyping a bit more digital. The Visio templates look interesting but lack any way to showcase the functionality of the proposed app. DENIM creates an interface that looks a little less polished (which is not a bad thing!) but can also demonstrate the desired functionality. Finally, I've added an article that talks a bit about methods of prototyping.
Visio - the interaction designer's nail gun