Nobody realises the importance of regular backup until they have a crash or theft of equipment and lose data! Think about it BEFORE this happens. Most companies have an overnight backup for data stored on the server, but make sure that you have provision for any data stored on the hard disks of PC's to be backed up as well.
Plan
Copy
The cheapest solution for an individual PC user who wants to back up data files would be back up to re-writeable CD's. If your PC does not have a re-writeable CD drive, you can easily install one yourself if you have a spare slot for it. If not, you can get an external re-writeable CD drive, but they cost more.
Alternatively you can back up to a USB data stick.
There are also lots of companies who offer an online backup service.
Schedule
Once you have decided what you are going to backup and how, make sure that you set up a regular backup schedule. The frequency of backup will depend on how much work you do, and how important it is for you to keep it secure. If a daily backup is too much for you, consider backing up at least weekly.