Ну как бы можно удалить лишние, уже не нужные в системе пакеты + очистить локальный репозиторий.
clean
clean clears out the local repository of retrieved package files.
It removes everything but the lock file from
/var/cache/apt/archives/ and /var/cache/apt/archives/partial/. When
APT is used as a dselect(8) method, clean is run automatically.
Those who do not use dselect will likely want to run apt-get clean
from time to time to free up disk space.
autoclean
Like clean, autoclean clears out the local repository of retrieved
package files. The difference is that it only removes package files
that can no longer be downloaded, and are largely useless. This
allows a cache to be maintained over a long period without it
growing out of control. The configuration option
APT::Clean-Installed will prevent installed packages from being
erased if it is set to off.
autoremove
autoremove is used to remove packages that were automatically
installed to satisfy dependencies for some package and that are no
more needed.
Т.е. сначала удалить неиспользуемые приложения, затем удалить то, что они притянули за собой:
sudo apt-get autoremove
а потом очистить локальный реп:
sudo apt-get clean
У меня на Eee Pc 900 под / было выделено 4Гб... Только так и справлялся...