It is not enough to have your server and service up and running. The server must be maintained and monitored even when everything seems to be fine. This includes security auditing as well as keeping an eye on the amount of remaining unused disk space, available RAM, the system's load, etc.
If these chores are forgotten, sooner or later the system will crash, either because it has run out of free disk space, all available RAM has been used and the system has started to swap heavily, or it has been broken into. The last issue is much too broad for this book's scope, but the others are quite easily addressed if you follow our advice.
Particular systems might require maintenance chores that are not covered here, but this section highlights some of the most important general tasks.