Setting up a local cache is a smart idea if you’re going to be running a VM server, whether it be KVM, VirtualBox, or any other option really. I assume the same instructions will work with Debian, but I haven’t tried.
First up - install apt-cacher-ng.
apt-get install apt-cacher-ng
Add this to the bottom of your /etc/services:
apt-cacher 3142/tcp apt-cacher 3142/udp
Edit /etc/default/apt-cacher and set AUTOSTART to 1.
See this post in the ubuntu forums for more details.
Restart the service.
Bam! You should now have a fully working apt-cacher-ng service, which will make your updates a ton faster. It will also make it easier for you to bootstrap new VMs if you’re using vmbuilder.