There are many "variables" with backups. Typically media needs to be changed daily. Software and data on the server is always changing, so it is possible for the size of stored data to change significantly from one backup to the next. Sometimes users can forget to close all applications at the end of the day, resulting in certain documents being left "open" and potentially unavailable to be archived by the backup job.

Commercial backup systems produce reports, typically a job log (for each backup) and an activity log (backup system overall), whenever a backup job is incomplete these logs are the first point of call in understanding why.

The summary report for each backup should be configured to print or e-mail daily to the person with primary responsibility for the backup system. If a backup job does not complete without errors, the backup system should also be configured to send a copy of the report to management and your IT support provider (if you have a support plan in place).

If using a tape drive, regularly using a cleaning tape will maintain the tape heads and will ensure trouble-free operation of the tape drive.

Backup media should be rotated so that at any time certain media is held off-site. In a worst case scenario where say there is a fire at the office, the backup's won't be of much use if they are destroyed along with the server.

It is also important to periodically verify that the backup is working properly by performing a test restore of selected data files (make sure the test restore doesn't overwrite production data - i.e. restore to a different destination). Then open the files to make sure that the information is intact.