We want to use our own local mirror managed by us for control the software versions that we use. We want to use software more updated than the stable versions, but also we don't want on critical moments a software update breaks Debian.

* First backup your current sources list

  mv /etc/apt/sources.list /etc/apt/sources.list.bak 

* Now open the sources list using your favorite text editor

  nano /etc/apt/sources.list
  

* Add the next line based on your Debian version, e.g: if you are using testing, add only the line that have testing on it

  deb http://arcoslab.org/debian/ stable main non-free contrib
  deb http://arcoslab.org/debian/ testing main non-free contrib
  deb http://arcoslab.org/debian/ unstable main non-free contrib

* Save the source list

* Update local sources repositories

  apt-get update -o Acquire::Check-Valid-Until=false

* Now you can use our local mirror

* Restore your backup sources file

  mv /etc/apt/sources.list /etc/apt/sources.list.bak

* Update local sources repositories

  apt-get update -o Acquire::Check-Valid-Until=false