My normal settings structure is as follows:

  * settings/__init__.py
  * settings/base.py
  * settings/development.py
  * settings/staging.py.
  * settings/production.py
  * settings/credentials.py

This allows me to have a base settings file which the others inherit from, and all settings are tracked in git apart from passwords which are stored in credentials.py.

Unlike some other approaches, installed apps are just defined once (in the base), but apps can be added/removed for each environment.