dpkg --get-selections

Source: How-To Geek - Show the List of Installed Packages on Ubuntu or Debian

Tested on stretch.

To change the apt-get sources.list to only use the ftp.nluug.nl mirror:

#deb http://ftp.nl.debian.org/debian stretch main contrib
deb https://ftp.nluug.nl/debian stretch main contrib

# security updates
#deb http://security.debian.org stretch/updates main contrib
deb https://ftp.nluug.nl/debian stretch-updates main contrib

Cleans out the apt cache in /var/cache/apt/archives/.

apt-get clean

Source:Ask Ubuntu - Is it possible to clean the APT cache from the command line?

For deleting packages that were leftover from previous releases. In this example from debian 7 and 8 on a debian 9 system.

Test removal:

apt-get remove --dry-run `dpkg -l | egrep "deb8|deb7" | awk  '{print $2 " "}' | tr -d "\n"`

Actually remove:

apt-get remove `dpkg -l | egrep "deb8|deb7" | awk  '{print $2 " "}' | tr -d "\n"`

For when apt for some reason thinks it can start using IPv6 in an environment with only IPv4 connectivity.

echo "Acquire::ForceIPv4 \"True\";" > /etc/apt/apt.conf.d/ForceIPv4