Backports cannot be tested as extensively as Debian stable, and backports are provided on an as-is basis, with risk of incompatibilities with other components in Debian stable. Use with care! According to Debian documentation, it is therefore recommended to select single backported packages that fit your needs, and not use all available backports.

Add backports to your /etc/apt/sources.list

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

Again, it is not a recommended configuration to use backports by default for all packages: it could break your system and delete all your data.

Are you still sure to do so? To use backported packages by default, add the following lines to /etc/apt/preferences

Package: *
Pin: release a=wheezy-backports
Pin-Priority: 600

Then, as usual apt-get update apt-get update apt-get dist-upgrade