Exclude packages from `apt-get upgrade`

To hold a package:

In short: sudo apt-mark hold <package>

or

echo <package> hold | sudo dpkg --set-selections

To unhold a package:

In short: sudo apt-mark unhold <package>

or

echo <package> install | sudo dpkg --set-selections

Check out "Introduction to Holding Packages" in the Ubuntu's Pinning Howto