Get a nearly fresh debian install without reinstalling

I was recently asked how to get rid of the old and unused packages without having to reinstall?

Debian have the mechanisms to deal with this and more. Unfortunately for new people, its not as automated and a little more obscure that i would like

Anyway, heres what i would do:

# apt-mark showmanual
# apt-mark auto <packages you dont recognize>
# apt purge <packages you recognize but dont want anymore>
# apt autoremove --purge