apt: how to avoid double downloads ?

["Peter L. Schroeder" <p.l.schroeder@gmx.de>]

Being a Debian-newbie I am fascinated of apt and want to
use it on both Debian-boxes in my home-net. 
I access a german ftp-site and it works quite well.
(Here is my /etc/apt/sources.list in addition to 
 Todd Suess´ recently  published one:
  deb ftp://ftp.de.debian.org/debian stable main contrib non-free
  deb ftp://ftp.de.debian.org/debian-non-US stable non-US
  deb ftp://ftp.de.debian.org/debian/dists proposed-updates/
)

What is a useful configuration in apt.conf and/or anywhere else
to avoid downloading packages twice if I run
apt-get first on one and then on the other box ?

I´ve tried the ftp::proxy section but did not
succeed. Can squid act as ftp-proxy-cache at all? If yes:
how do I have to write the ftp::proxy-section of apt.conf ?

Or is there a more suitable configuration you recommend -- using
apt-cache and nfs for example? 

Regards
Peter