The reason the codebase has got into a mess is because we failed to execute on important principles such as:

1. Take account of coupling and cohesion.
2. Be clear about people's roles and responsibilities to avoid unqualified or inappropriate decision making.
3. Clarity and simplicity of roles and responsibilities in design elements.
4. Maintain modular, well-isolated code and conceptual integrity.
5. Avoid shared data-schemas or integration via the database.
6. Make the software testable and maintain the tests.
7. Select technology based on appropriate design work.
8. No broken windows.
9. Track and maintain appropriate metrics.
10. Review projects to identify and disseminate useful lessons to developers, architects and customers.
11. Account for the operational aspects of our software in requirements and design.
12. Review to ensure code aligns with appropriate design principles.
13. Surface, balance and mitigate risks.

Get general engineering practices (back) into software development! Now!

Reference: http://www.dancres.org/blitzblog/2008/12/02/remodelling/