First you must add some sources in /etc/apt/sources.list.d/testing.list (create it if it does not exists).

deb http://mirror.peer1.net/debian/ testing main contrib non-free
deb http://security.debian.org/ testing/updates main contrib

Then remove the package if it was already installed and reinstall the dev version with this command:

aptitude -t testing install python-django

To see if it has worked you can type this command:

aptitude -t testing show python-django

You should see "Django-1.1.1".

Thanks Lastcall_ :)