APT pinning and downgrading

[Gregory Seidman <gss+debian@cs.brown.edu>]

I read http://www.debian.org/doc/manuals/apt-howto/ch-apt-get.en.html
(particularly section 3.10), and came up with the following process to
downgrade from unstable to testing:

1) in my /etc/apt/preferences (which didn't exist until I created it):

Package: *
Pin: release a=testing
Priority: 1001

2) in my /etc/apt/apt.conf (which didn't exist until I created it):

APT::Default-Release "testing";

3) apt-get upgrade -u

Unfortunately, this doesn't do anything. It tells me about the packages I
have on hold (just those I compiled myself with g++ 3.2), and that's it.
What am I doing wrong?

--Greg