Generally I prefer to install Debian using minimal CD so Apt-Cacher NG is an integral part of this process as I don't like to wait during installation process.
To define HTTP proxy (
http://192.168.1.102:3142/ in this example) add the following entry to the kernel parameters after pressing TAB key on the Install option in the installer boot menu.
Please note that provided HTTP proxy will be stored in /etc/apt/apt.conf configuration file.
after_install$ cat /etc/apt/apt.conf