Add the following to your .bashrc:
# -------------------------------------------------------------
# Variables for apt-get
export HTTP_PROXY="http://myproxy.mydomain.com:myportnumber/"
export http_proxy="http://myproxy.mydomain.com:myportnumber/"
export FTP_PROXY="http://myproxy.mydomain.com:myportnumber/"
export ftp_proxy="http://myproxy.mydomain.com:myportnumber/"
# -------------------------------------------------------------
Enjoy!