mixing stable and testing packages

Sometimes when sticking with a Debian stable version, it can be useful to just update one or two packages to a testing or even unstable version, because of new features.

The ideal way to do this is to use backports, because these have been tested (and patched if necessary) to work with the stable versions of other packages. The backports repository is added to /etc/apt/sources.list:
Backport packages have a lower priority by default, but the backport version can be forced by using apt-get:
In cases where the package is not available as a backport, it is possible to mix testing packages with stable ones. There is a risk of incompatibilities, but many user applications will work fine.

First the testing repositories are added to /etc/apt/sources.list, alongside of the repositories. Then the default release is set to 'stable', as normally the testing versions will override, and the whole system would be updated. In older versions of Debian, this was in /etc/apt/apt.conf; now the configuration can be set in a file in /etc/apt/apt.conf.d, for example in 70debconf:
Since the default is set to 'stable', a testing version of an application is forced using apt-get: