Usually you define an interface for anything that you want to be overridable and provide some way for consumers to provide their own implementations. Then seal your own implementations.

Then you only need to design your interfaces in such a way that they can easily be implemented by composition.

This is the approach taken in ASP.Net and in WCF; although the latter does a really bad job at making their interfaces user friendly (for even tiny changes you have to re-implement a lot of functionality - which could have been avoided by better interface design).