add repository

If you get error:
E: Dynamic MMap ran out of room

edit /etc/apt/apt.conf to contain this line:
APT::Cache-Limit "10000000";

rerun update.

edit /etc/apt/preferences to include these lines:

Package: *
Pin: release a=stable
Pin-Priority: 700Package: *
Pin: release a=testing
Pin-Priority: 650

apt-get prefers options with higher pin values, so this prioritizes stable over testing.
use apt-get install -t testing [packagename]