Backup
Backing up data is something many don't consider untill after they have lost vital data or paid a large sum for data recovery to be performed. By avoiding backing up data that that could have taken a couple of minutes a day you might have to spend weeks re-creating the lost data.
If you are reading this you are hopefully already aware of the advantages of having proper backups, so let us skip to suggesting some approaches.
DVD-R and CD-R
Having a DVD-Recordable or a set of CD-Recordable discs is an easy way to back up data if the amount isn't too great. The average CD-R can hold 700MB of data, while a DVD±R can hold 4.5 GB of data (single layer). This should be sufficient to back up the data for most users, including documents, emails et cetera.
This is also a very nice approach in the event that you want to store the data in a remote location, say in the event of a break-in or a fire. It can easily be placed in a bank deposit box or at a trusted friend's house ( if you are worried about the content on the disk falling in the wrong hands, please read our securing data section)
External harddrive
If you have more data, or feel that it is cumbersome to record and keep track of the DVD or CDs you can (in addition?) purchase an external Hard Disk Drive that use a high speed connection such as USB2 or Firewire. You can either copy files manually or use an automated tool to copy data at spesific times.
Network backups
If you have multiple computers, for instance a laptop and a desktop it will probably make sense to copy data from the laptop to the desktop (provided the desktop has enough storage). Loosing the laptop's data either by it getting stolen, by dropping it to the ground or because it overheat (personal experience) can be destructive unless you have proper backups.
You can either use a feature such as shared folders in Microsoft Windows , Network FileSystem (NFS) or my personal favourite rsync to back up the data.
rsync is an utillity that follow most GNU/Linux distributions, but is also available for Microsoft Windows. It is a very powerful application, but with power come complexity, so it is not the a tool recommended for most users. If you do, however, want to read more about using rsync on windows, try this collection of links.