Problem of communication as code changes over time

Two simple and fundamental steps that can be taken to avoid the message being lost over time, comes to mind.

1) Write JavaDoc: For example, for a method, the javadoc should contain the 'intention'; what the method is supposed to do, plus it's limitations. This should make it easier for a new developer to get started with the method and make any changes without messing up the expected outcome.

2) Conform to the fundamentals of Object-orientation. Overload/override instead of changing method signatures. Conform to interfaces (interfaces must be implemented in the first place). These are just a couple of examples and there are a whole lot of them.