sudo sh -c 'echo "deb http://download.virtualbox.org/virtualbox/debian trusty contrib" >> /etc/apt/sources.list.d/virtualbox.list'
According to your distribution, replace 'xenial' by 'vivid', 'utopic', 'trusty', 'raring', 'quantal', 'precise', 'lucid', 'jessie', 'wheezy', or 'squeeze'.
wget -q https://www.virtualbox.org/download/oracle_vbox_2016.asc -O- | sudo apt-key add -
wget -q https://www.virtualbox.org/download/oracle_vbox.asc -O- | sudo apt-key add -