SBC Games wrote a series of tutorials on building games that are easy to maintain: "Many tutorials show you how to build a game. Let’s say, you already have one. And because it is good and catchy, sponsors are interested in it. Your nice and clean code is step by step polluted with sponsor specific APIs.
Maybe, even worse, you are maintaining separate project for each sponsor and fixing bug in one file needs some synchronization into other projects. Or game is built on your libraries and you add some feature, you would like to propagate into all your games. Or you need easy way how to parameterize game globally or individually for specific sponsor.
In this series I would like to address these issues and build game, that is easy to maintain. With one change of parameter, you can build for another sponsor or you can easily parametrize it without rebuilding, etc."