Hey, I have been out of the loop so forgive you me if this is old news. :-$

The Microsoft Patterns an Practices team has had two really interesting releases lately. The purpose of them both is to help you avoid writing monolithic applications. In true agile spirit, the aim is to write only as much as necessary at the time.

First out, the Composite Application Guidance for WPF. I attended a briefing on this by Dotway's George Bolsch, who is on the advisory board of this particular CAG. The idea is to provide software development teams with the tools needed to build their WPF apps in a highly modular way. The focus in this case is not the application itself but the construction process. By applying CAG for WPF, you can split the development across multiple teams and then seamlessly compose those pieces into an application.

The second release on my mind is the Managed Extensibility Framework. Unlike CAG for WPF, which focuses on composition, MEF provides application extensibility. Team vs. application focus in other words. MEF can, in its simplest form, be considered a generic plug-in mechanism but from my perspective, it provides an enhanced support for agile development principles. If you make sure that you have implemented the proper extension points and that the application's core features are in place, you can then add functionality in the form of extensions without having to recompile the core. There is also an MSDN Code Gallery Area for MEF.

I really didn't think of it before I started writing but the apparent direction of thoughts after mentioning these two releases would be to also consider implementing some kind of Dependency Injection / Inversion of Control framework. Obviously, the minds of the P&P team wandered that way as well and they have subsequently released their first attempt at a DI container, called Unity. Coincidently, CAG for WPF comes with out-of-the-box support for Unity.

These are all techniques that you should consider using when you develop .NET and in particular WPF applications. The minimum engagement is to at least know as much about them that you can make an well grounded decision to leave them out of your solution rather than just missing them by accident.