Let us suppose that you are running Debian stable, but there are some packages that you would like from the backports archive. However, you don’t want all backports replacements to be brought in.

In etc/apt/sources.list.d/backposts.list add the backports repo. As I write this, Jessie is stable, so:

deb http://http.debian.net/debian jessie-backports main contrib non-free

Note that the source lists must end with .list or they will be ignored.

In etc/apt/preferences.d/priorities:

Package: *
Pin: release a=jessie
Pin-Priority: 900

Package: *
Pin: release a=jessie-backports
Pin-Priority: 50

and run apt-get update

There are magic levels for the numbers in the priority field. They are explained, at length, in man apt_preferences. The highlights:

•  P >= 1000
      causes a version to be installed even if this constitutes a
      downgrade of the package

•  990 <= P < 1000
      causes a version to be installed even if it does not come from the
      target release, unless the installed version is more recent

•  500 <= P < 990
      causes a version to be installed unless there is a version
      available belonging to the target release or the installed version
      is more recent

•  100 <= P < 500
      causes a version to be installed unless there is a version
      available belonging to some other distribution or the installed
      version is more recent

•  0 < P < 100
      causes a version to be installed only if there is no installed
      version of the package

•  P < 0
      prevents the version from being installed

Now, to install a single backported package, apt-get install packagename/jessie-backports, and to install a backported package plus any dependencies which are also backported, use apt-get -t jessie-backports packagename