I put in a lot of effort yesterday studying the man pages for apt and apt_preferences as well as the APT HOWTO which is buried deep in the Debian website.
To no avail. apt turned up its nose at my feeble attempt to write a preferences file. I usually beat my head against the brick wall until I figure out obscure documentation but on this one, the patience isn't forthcoming. Rather than ask help, if someone has a smoothly functioning /etc/apt/preferences file which they would be willing to share, please e-mail it to me ([EMAIL PROTECTED]) and I will use it to figure out where I went wrong on the concepts.