apt-get update
apt-get -u upgrade
apt-get -u dist-upgrade
apt-get clean or apt-get autoclean

Get to know and love these commands as they will keep your machine in tip-top working order – hopefully. The first has to be run to update your apt database with the newest sources. The second is generally used to upgrade your packages and essentially provides a safe upgrade. The third is useful if you have a lot of packages that have new dependencies and you just want to upgrade everything, such as when you upgrade from stable to testing, but it can be useful at other times too. The last two clean out your apt cache of downloaded files. You need to run them periodically to ensure your disk doesn't fill up. The first is pretty harsh the second less so. If you have a big Internet connection the first is the most efficient on space.

Should you suffer a fate worse then death - a full disk - log in as root (a small amount of drive space is reserved for root and so you should be able to log in) and run the command

du -h -x --max-depth=1

in the root directory. It will tell you how much space each directory below the current directory is using. I found that cleaning the apt cache freed up huge amounts of space but your mileage may vary

Since Apt 0.6 the main repositories have been digitally signed. If you don't want warnings everytime you try and update a package you should download and install the key or switch off the warnings. To download and install the key:

• Download the key from this page http://www.debian.org/releases/ (it's towards the bottom of the page).
• Install gnupg (which deals with the signature checking and various other security bits).
• Run "cat ziyi_key_2006.asc | gpg --import" as root to tell gnupg about the key (youmight need to change the key name).
• Run "apt-key add ziyi_key_2006.asc" to tell apt about the key.

To simply ignore the warnings add:

APT::Get::AllowUnauthenticated 1;

To /etc/apt/apt.conf creating the file if it is not present.