Acquire::http::Proxy "http://login:password@yourproxyaddress:port";

Acquire::http::Proxy "http://bob:123bob@bobproxy:8070";

ftp_proxy="ftp://username:password@proxy.com:8080/"
http_proxy="http://username:password@proxy.com:8080/"
https_proxy="https://username:password@proxy.com:8080/"

export ftp_proxy="ftp://username:password@proxy.com:8080/"
export http_proxy="http://username:password@proxy.com:8080/"
export https_proxy="https://username:password@proxy.com:8080/"

sudo apt-get install gconf-editor

gconf-editor