DOS attacks are becoming very common these days. There are many different levels of DOS attack and monitoring your system and trying to prevent them all can be a full-time job. However for the causal user you can easily add some basic DOS protection without much effort, especially if you are using Fedora or CentOS.
Fedora and CentOS both have mod_evasive in their main repositories so to install this you just need to run:
yum install mod_evasive
This mod comes basically ready to run without any modification. I like to make a couple of changes to the settings so I can receive notices when a block is taking place. To do this you just need to go to your apache conf.d directory and edit the mod_evasive.conf file. To recieve email notifications when someone is blocked just enter your email address in the field DOSEmailNotify. All of the other settings are fine the way they are.