From the apt-get manpage:
echo libc6 hold | dpkg --set-selections
will hold back libc6, useful when you want to hold back a package from upgrading if you call:
An even easier way from http://www.debian.org/doc/FAQ/ch-pkg_basics.en.html
With aptitude, you can hold a package using aptitude hold package_name
and remove the hold with aptitude unhold package_name
Hmmmm, the aptitude command isn't working for me. I wonder if I'm missing something. Nope, doesn't seem like it. I've never used aptitude before.