The post from earlier today provides information on setting up basic data backup for your computers. For me it's got flaws. If the building burns down, your computer(s) and NAS are gone - along with your data.
My backup strategy has been to use basic approach outlined in the above link to provide continous protection of all machines and write complete images of critical systems and data to two additional single (large) drives. One of these drives goes in a fireproof safe when we are away, and the other to a neighbour. If you followed the solution in the previous post, you can do something like this too, by connecting an additional USB hard drive to your NAS and setting up the NAS to backup itself to this hard drive every week. OK, but you have to remember to do it. And the copy in the fireproof safe is by definition always out of date.
There is a better way that is dead simple!
Ideally, you want to have continuous backup running to somewhere offsite. But in countries like Australia, data caps make this impossibly expensive. Even in the US, the data storage limits for commercial solutions may be expensive if you really want to save all your TB of media.
However, you can in many cases create an "onsite offsite" solution. Many of us have a garage, studio or shed separate from our main dwelling. The idea is to place a NAS in that location, linked either via ethernet cable or wirelessly. Wireless repeaters cost only about $60 each, so even if you have to set up a dedicated high speed wireless link, it will cost less than $150. If your main building suffers a disaster, then the NAS is there - and vice versa. If you have no separate building - ask your neighbour if you can site a NAS / dedicated wireless link somewhere out of the way; and offer to return the favour.