Acquire::http::Proxy “http://proxy.adresse.de:port”;
Acquire::ftp::Proxy “http://proxy.adresse.de:port”;

oder die Langfassung (http://debiananwenderhandbuch.de/apt.conf.html):

// Options for the downloading routines
Acquire
{
Queue-Mode “host”; // host|access
Retries “0″;
Source-Symlinks “true”;

// HTTP method configuration
http
{
Proxy “http://127.0.0.1:3128″;
Proxy::http.us.debian.org “DIRECT”; // Specific per-host setting
Timeout “120″;
Pipeline-Depth “5″;

// Cache Control. Note these do not work with Squid 2.0.2
No-Cache “false”;
Max-Age “86400″; // 1 Day age on index files
No-Store “false”; // Prevent the cache from storing archives
};

ftp
{
Proxy “ftp://127.0.0.1/”;
Proxy::http.us.debian.org “DIRECT”; // Specific per-host setting

ProxyLogin
{
“USER $(PROXY_USER)”;
“PASS $(PROXY_PASS)”;
“USER $(SITE_USER)@$(SITE):$(SITE_PORT)”;
“PASS $(SITE_PASS)”;
};

Timeout “120″;

/* Passive mode control, proxy, non-proxy and per-host. Pasv mode
is prefered if possible */
Passive “true”;
Proxy::Passive “true”;
Passive::http.us.debian.org “true”; // Specific per-host setting
};