Use a tool like Microsoft SketchFlow, or make your prototype in some other language or platform, making it nearly impossible to integrate into main development.
There's also a joelonsoftware essay about showing screenshots and prototypes, where he makes unimplemented and unworked aspects appear obviously broken/unimplemented, making it clear where work still needs to be done.
Important Corollary Two. If you show a nonprogrammer a screen which has a user interface which is 100% beautiful, they will think the program is almost done.
What can you do about this? Once you understand the Iceberg Secret, it's easy to work with it. Understand that any demos you do in a darkened room with a projector are going to be all about pixels. If you can, build your UI in such a way that unfinished parts look unfinished. For example, use scrawls for the icons on the toolbar until the functionality is there. As you're building your web service, you may want to consider actually leaving out features from the home page until those features are built. That way people can watch the home page go from 3 commands to 20 commands as more things get built.
So, try making your prototypes in Photoshop instead of Visual Studio, or something along those lines.