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!
Add backports to your /etc/apt/sources.list
deb http://http.debian.net/debian wheezy-backports main contrib non-free
According to Debian documentation, it is therefore recommended to select single backported packages that fit your needs, and not use all available backports. 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