if u want to split ur settings.py
or use custom settings per site

lets say it looks like this:

'project/settings.py'

just create a folder named 'settings'.
create an '__init__.py'
copy the content of 'settings.py' to '__init__.py'
delete the ol' bastard named 'settings.py'

now it should look like

'project/settings/__init__.py'

if u want to put database settings in a seperate file.
create 'db.py' in 'settings' ...

'project/settings/__init__.py'
'project/settings/db.py'

and simply add this line to '__init__.py':
'from db import *'

to resuse an existing 'settings.py' simply import it in ur custom 'settingsxXXX.py' with:

'from settings import *'

test it with './manage.py runserver --settings settingsxXXX'