5.6. Care and Feeding
You'll want to install some form of monitoring for all of your systems. Section 5.6.2 lists several good open-source monitoring tools for both host-based and network-based monitoring. You should also take a look at monit as a means of watching your systems internally (meaning not from an external server the way nagios would do it). Others prefer to use runit to manage their processes reliably, and it should work well with monit.
You can install a few small pieces of software right away and get a massive security improvement.