Firstly edit or create the /etc/apt/apt.conf file and add this line;
vim /etc/apt/apt.conf
APT::Default-Release "testing";
Now we need to add the unstable repositories to the sources.list file.
vim /etc/apt/sources.list
Here is the sources.list file I am using at the time of writing.
# Testing Repository
deb http://ftp.au.debian.org/debian testing main contrib non-free
deb-src http://ftp.au.debian.org/debian testing main contrib non-free
deb http://ftp.debian.org/debian/ testing-updates main contrib non-free
deb-src http://ftp.debian.org/debian/ testing-updates main contrib non-free
deb http://security.debian.org/ testing/updates main contrib non-free
deb-src http://security.debian.org/ testing/updates main contrib non-free
# Unstable Repository
deb http://ftp.au.debian.org/debian unstable main contrib non-free
deb-src http://ftp.au.debian.org/debian unstable main contrib non-free
Lastly update the repositories.
apt-get update
Now if you want to install a package from the unstable distribution of Debian use the apt-get command with a couple of extra parameters This example is installing nodejs from unstable.
apt-get -t unstable install nodejs
