I work on a fairly large software system and over the years it has accumulated a lot of entropy. There is plenty of scope for refactoring but there is always pressures to build the next features upon what's already there. This adds to more entropy, because design choices for implementing new features are typically made by first accepting what's there and 'working-around' some earlier weaker design that is otherwise ripe for refactoring.
What are some ways to manage this kind of complexity and build functionality without substantially weakening the structure of the system further. I know this is a very broad question and the approaches/solutions depend on the particular software system at hand, but I am still hoping there are some generic ways to manage the problem I am highlighting.