I'm in the process of creating models for my CI project, and realized that certain functions within the models were getting repetitive. Using CI's ability to create my own custom libraries, I was able to create my own custom 'Model' which extends from the core 'Model' object. How this simple architecture has cleaned up my code is simply remarkable.
Rather than overwriting the main Model.php file with some of your own changes, he suggests creating a new library, a "parent model" that can be extended instead to provide some common functions that all of your application's models might need. For something a bit more complex, he also points to this library that extends the models to give it CakePHP-like functionality.