How to properly backup OS X Server (Mavericks)
I use Carbon Copy Cloner (actually Super Duper - but same idea) to back up my system drive to another drive.

Additionally, I have another CPU which acts as an Open Directory Replica in case the first one goes down.
(granted, this means you need 2 CPUs - so it's not the cheapest solution!)

You can find some more info about OD Replica here:
http://arstechnica.com/apple/2013/12/a-power-users-guide-to-os-x-server-mavericks-edition/2/