If (like me) you find yourself re-installing your machines a lot (and let's face it, that's what Foreman is for), then a package cache can save you a huge amount of data traffic. There are many ways to achieve this, but I'll describe how I'm using apt-cacher-ng to help me save data.
Apt-cacher-ng is particularly helpful since it can support almost any OS - the documentation has instructions for Debian, Ubuntu, Fedora, CentOS, OpenSUSE, and even Archlinux.
