I'm lazy, I use RAID 1. Then each week I remove one of the HDDs, add the spare and rebuid the volume. By rotating the 3 HDDs, I'm keeping the mirror for daily use, AND doing the weekly backup just in case of a catastrophic failure.
You only need RAID 1 if you must have the computer up at all times, like with businesses. If you're not worried about this, then just utilize a backup.
In this case, you can have two drives installed internally so you only backup media files and documents or you can have an external hard drive to do the same.
If you're terrified that something may happen to your data you can purchase a hard drive dock ($25 at geeks.com) and after backup remove the drive and store it somewhere else.
With each of these above options, though, your backup drive will still be susceptible to viruses, etc. Thus, the best backup option is offsite, where you pay a monthly fee to have it automatically backup during the night.
But for most users a second backup drive is adequate, but you must realize that if someone steals your computer or your house floods or catches fire you will lose everything. Thus, an offsite backup is the preferred method.
For businesses with important data, ie data that would cost $1000s plus to reproduce or can't be reproduced, I go with Raid 1, dual USB backups and off site backups.
Typically I see at most 100gb in most business settings. So a 1TB or 2TB drive could be setup to do daily Mon.-Fri. backup with a bi-weekly Saturday backup and a 1 month backup. If you swap those once a month or even week you can save pretty much all your data. Even if your backup gets corrupted you can go to a previous day and still be able to backup.
The off site backup can be used in the event your normal backup fails or gets a virus. So you end up having even more fail safes built in. Including the case of a catastrophe happening in your office.
For home users I suggest a weekly automatic backup to USB drive and off site for things that cannot be replaced, family video and photos etc. I often just suggest burning a copy to DVD and sending it to a parents house to store. But online storage is becoming so cheap that is not needed anymore. Also one of the last things to burn is your freezer. In case of a fire or flood your DVD copies in the freezer should survive. An average disc can handle -40C to 70C though some are only able to handle -5C to 55C so check your manufacturer prior to buying for this purpose.