How can I prevent aptitude from upgrading me past lenny? (Debian Linux)

Question

I can't seem to stop aptitude from upgrading all my packages to testing or beyond.

I'm running Debian lenny, and generally I want to stick with packages from lenny unless I explicitly say otherwise. For particular features I may upgrade a particular package to a version from testing or experimental, but in general I want to stick to the lenny versions. In my /etc/apt/apt.conf I have uttered

APT::Default-Release "lenny";

and my /etc/apt/preferences is listed below.

Problem: whenever I hit the U key in aptitude, it wants to upgrade all my packages at least to testing.

Question: how can I arrange that the U key in aptitude installs only new versions in lenny, not newer versions from testing or elsewhere?

My /etc/apt/preferences (where I freely admit I don't know what I'm doing) is

Package: *
Pin: release a=stable
Pin-Priority: 900

Package: *
Pin: release a=testing
Pin-Priority: 90

Package: *
Pin: release a=experimental
Pin-Priority: 89

Package: *
Pin: release a=gutsy
Pin-Priority: 88

Package: *
Pin: release a=unstable
Pin-Priority: 50

Answer 1

Set APT::Default-Release to "stable", not "lenny". The Release file talks in terms of release stages, not release codenames.

Answer 2

Check your /etc/apt/sources.list. Try using release codenames.

i.e. deb http://ftp.ie.debian.org/debian/ lenny main contrib non-free