The setup: Winform/ASP.NET MVC projects. Learning NHibernate SQL-Server driven apps

I work with clients that have no idea how to model an application. That's what I'm for. However, we have lots of conflicts with validation, mis-understandings, etc.

For example, the client will ask for an order entry screen. The screen should require a "product". That's fine and dandy. However, the client didn't know to tell me that the user can't order a product of "Class A" unless it's Tuesday.

Or, they need a time entry screen. 2 days before it's rolled into production, they casually forgot to mention that certain activities are only valid for certain situations. These situations being a week of coding.

That's of course, some crude examples (not by much!). But the problem is getting these non-technical clients to layout their business logic. They somehow didn't realize that the "Class A" problem would come up two weeks later, etc.

I'm all for agile programming but is there an easy way to somehow make business logic like this extremely easy to implement and change on almost a daily basis?

I of course am splitting the project into hopefully intelligent pieces, using NHibernate, etc. But making this BI logic so dynamic is really making it hard to project timelines, etc.

Any suggestions? I know there will never be a perfect client (or a perfect provider) but how do you guys deal with the constant mis-understandings?

Thanks.