Somebody mentioned apt pinning, which will achieve what you want. But the easier way is to put this in /etc/apt/apt.conf:
APT::Default-Release "squeeze";
Test this by running 'apt-get update && apt-get upgrade'. You don't have to go through with the upgrade, just look and see what is being upgraded. You can also run 'apt-cache policy somepackagename' to see the installed version of a package and the available versions, along with their pin value.
Substitute "squeeze" for "wheezy" if you want Debian Stable (it looks like you're currently running Squeeze, which is the old stable).
-Rob