Backups are critical. In addition to the various good methods described above of backing up either files, or applications or your OS or some combination of data, apps, or OS, two suggestions.
First:
Periodically check to see if the backed up DVDs, or images, or mirrored drives really work. That is, can you retrieve the data from them?
Why?
If you have corrupted files on your source drive and backup those files, they will still be corrupt on the backup method you have selected.
Second. Word processing programs, spread sheets and databases write data in proprietary formats. This is fine, but you cannot always exchange data with someone else. More importantly, if you have corrupt files, data recovery is more difficult.
Solution: these same programs have options to export or save the data in somewhat more neutral formats such as CSV, tab delimited, that show up as file extensions with (.tab, .txt and .csv)
Periodically do so, and back up that data as well.
You may loose a lot of formatting, but the bulk of the files and information will be there.
Then, should all hell break loose and you lose program files and data, someone using a different word processor, or spread sheet or database program can still import your data and you can start again.