Ubuntu:

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 00000000

Put a package on hold

echo "package hold" | dpkg --set-selections

Remove the hold

echo "package install" | dpkg --set-selections

Store the list of software to a file called /backup/package-selections (restore see aptitude):

dpkg --get-selections >/backup/package-selections

Hold a package using

aptitude hold package_name

Remove the hold with

aptitude unhold package_name

Restore installed software (store see dpkg):

aptitude install $(cat /backup/package-selections | awk '{print $1}')