{{{ man 5 apt_preferences test }}}
See also [http://www.debian.org/doc/manuals/apt-howto/ch-apt-get.en.html Apt-Howto] (Section 3.10) and AptPinning. -- StephenHargrove
An example of /etc/apt/preferences file
Package: * Pin: release o=Debian,a=testing Pin-Priority: 900
Package: * Pin: release o=Debian,a=unstable Pin-Priority: 300
Package: * Pin: release o=Debian Pin-Priority: -1