First, using the Debian Backports instructions as a reference, I added squeeze-backports to my sources.

To manage my growing list of repositories, instead of cramming everything into /etc/apt/sources.list (still not the worst idea, but I’m going another way) I have been creating separate files in /etc/apt/sources.list.d/ for all of my various repositories, and I did the same for Debian Backports, entering this repository line:

deb http://backports.debian.org/debian-backports squeeze-backports main