Holding

There are different ways of holding back packages: with dpkg, apt, dselect, aptitude or Synaptic.

dpkg

Put a package on hold:

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

Remove the hold:

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

Display the status of all your packages:

dpkg --get-selections

Display the status of a single package:

dpkg --get-selections <package-name>

Show all packages on hold:

dpkg --get-selections | grep "\<hold$"

apt

Hold a package:

sudo apt-mark hold <package-name>

Remove the hold:

sudo apt-mark unhold <package-name>

Show all packages on hold:

sudo apt-mark showhold

dselect

With dselect, enter the [S]elect screen, find the package you wish to hold in its present state and press = or H. The changes will take effect immediately after exiting the [S]elect screen.

The following approaches are limited in that locking/holding a package within aptitude or synaptic doesn't affect apt-get/apt.

aptitude

Hold a package:

sudo aptitude hold <package-name>

Remove the hold:

sudo aptitude unhold <package-name>

Locking with Synaptic Package Manager

Go to Synaptic Package Manager (System > Administration > Synaptic Package Manager).

Click the search button and type the package name.

When you find the package, select it and go to the Package menu and select Lock Version.

That package will now not show in the update manager and will not be updated.