OK, this section should be comparatively short, simple and straightforward compared to the above, but no less important.
The very first thing after a new install you should check your distribution's updates and security notices and apply all patches . Only a year old you say? That's a long time actually, and not current enough to be safe. Only a few months or few weeks? Check anyway. A day or two? Better safe than sorry. It is quite possible that security updates have been released during the pre-release phase of the development and release cycle. If you can't take this step, disable any publicly accessible services until you can.
Linux distributions are not static entities. They are updated with new, patched packages as the need arises. The updates are just as important as the original installation. Even more so, since they are fixes. Sometimes these updates are bug fixes, but quite often they are security fixes because some hole has been discovered. Such "holes" are immediately known to the cracker community, and they are quick to exploit them on a large scale. Once the hole is known, it is quite simple to get in through it, and there will be many out there looking for it. And Linux developers are also equally quick to provide fixes. Sometimes the same day as the hole has become known!
Keeping all installed packages current with your release is one of the most important steps you can take in maintaining a secure system. It can not be emphasized enough that all installed packages should be kept updated -- not just the ones you use. If this is burdensome, consider uninstalling any unused packages. Actually this is a good idea anyway.
But where to get this information in a timely fashion? There are a number of web sites that offer the latest security news. There are also a number of mailing lists dedicated to this topic. In fact, your vendor most likely has such a list where vulnerabilities and the corresponding fix is announced. This is an excellent way to stay abreast of issues effecting your release, and is highly recommended. http://linuxsecurity.com is a good site for Linux only issues. They also have weekly newsletters available: http://www.linuxsecurity.com/general/newsletter.html.
Also, many distributions have utilities that will automatically update your installed packages via ftp. This can be run as a cron job on a regular basis and is a painless way to go if you have ready Internet access.
This is not a one time process -- it is ongoing. It is important to stay current. So watch those security notices. And subscribe to your vendor's security mailing list today! If you have cable modem, DSL, or other full time connection, there is no excuse not to do this religiously. All distributions make this easy enough!
One last note: any time a new package is installed, there is also a chance that a new or revised configuration has been installed as well. Which means that if this package is a server of some kind, it may be enabled as a result of the update. This is bad manners, but it can happen, so be sure to run netstat or comparable to verify your system is where you want it after any updates or system changes. In fact, do it periodically even if there are no such changes.
It is very simple: make sure your Linux installation is current. Check with your vendor for what updated packages may be available. There is nothing wrong with running an older release, just so the packages in it are updated according to what your vendor has made available since the initial release. At least as long as your vendor is still supporting the release and updates are still being provided.