Another option is the File attribute. An example is provided in the following blog. I think this attribute has more going for it than the configSource attribute.
Default + Optional Override
The file attribute allows you to keep default settings in your web.config. If .NET finds the specified configuration specified by the file attribute, it overrides your defaults. This provides you with the option to decouple your configuration from web.config:
If asp.net finds a "user.config" in my application directory, it uses those settings.
Available in ASP.NET 1.1
Given the file attribute's availability in asp.net 1.1, I don't see the added value that the configSource attribute provides in asp.net 2.0. Comments?
File Must Reside within the Application's Physical Directory Structure
To further decouple the application from configuration, I tried specifying the file in a virtual location. This would allow me to stage configuration updates separately from application updates. Like the configSource attribute, it was no dice. Rats.