From: http://stackoverflow.com/questions/512906
Add the following to /etc/apt/preferences:
Package: *
Pin: release a=testing
Pin-Priority: 200
200 means that new packages in testing still override local packages that are not in stable (local is always 100), but not ones that are in the stable repo as well.
To install packages from testing:
$ apt-get install -t testing <some_package>