I've found this to be the easiest and quickest way when I setup a lean VM with a debian based distro (like debian-cloud, Ubuntu server, Lubuntu, etc) and/or Raspberry Pi (raspbian):
Edit apt sources list with:
sudo nano /etc/apt/sources.list
Add the following lines at the bottom:
deb http://ftp.debian.org/debian unstable main
deb-src http://ftp.debian.org/debian unstable main