Kristopher Wilson has a new post to his site talking about something that could be very difficult with an existing application (and a good starting place for a new one) - decoupling from the framework. He advocates that your application shouldn't be an extension of the framework, more so a user of it to make it potentially easy to replace.
We spend a lot of time discussing and analyzing the features and merits of several frameworks, trying very hard to make sure we find the perfect one to use for our project. Rightfully so: picking the wrong framework can lead to a slew of issues down the road in terms of maintenance and scalability. [...] We also spent a considerable amount of effort making sure that there is minimal amount of coupling within our code. Strong coupling leads to problems testing, adapting, refactoring and reusing code. What if we applied that same principal to dealing with whatever framework we're using?
He goes on to look at the "framework is not your application" concept and fleshes it out with examples of it applied to a few different topics: Controllers, Models and ORMs. He also shows how, through the use of something like Doctrine's EntityManager, you can easily abstract things out so the internals of the application can easily split the application and framework.