The five SOLID design principles can be applied to .Net development in the following ways:
Each class and method should have exactly one purpose.
Use generic or abstract classes where future extension may be necessary rather than continually modifying an inherited base class.
Use Code Contracts where appropriate for improved testability. (ccrewrite for run-time testing and cccheck for compile-time testing)
With MVC and MVVM you’re pushed into this naturally which is good. Just make sure you don’t have any business logic in your views, view models or controllers.
Use factories, containers and interfaces where appropriate to make use of libraries in situations where it’s useful not to have to reference them directly.