On keeping /var/cache/apt/archives empty
Steven Hanley was pondering how to keep /var/cache/apt/archives empty on the mirror that I help administer.
This sounds vaguely similar to mounting the /usr filesystem read-write at the start of an APT run and read-only again at the end. (A practice I used to believe in, but due to various package upgrades making /usr busy for no good reason, and it artificially inflating the maximal mount-count and prematurely causing a fsck at boot, I've discontinued)
So, putting
DPkg::Post-Invoke { "apt-get clean"; };
in /etc/apt/apt.conf (or in a file in /etc/apt/apt.conf.d) ought to do the trick.