This is a topic that has been well-covered here.
Some threads to get you started (with further references) include:
- How do you structure a large Django project for scalability and maintainability?
- Fat models, thin views, what about tasks
- Business logic: what's the effort to switch between approaches?
If you follow through to all the threads they reference, and then to their references (and so on), you’ll get a pretty comprehesive view of what people are doing. (Most all of them do talk about people’s actual implementation and not just theoretical advice.)