"There is nothing so useless as doing efficiently that which should not be done at all. "
Peter Drucker
In order to maintain a proper backup system, you'll need to use more than one set of backup tapes or disks.
This is probably not what you wanted to hear. Tapes and disks with a suitable capacity for performing backups are not cheap and buying enough for backing up your data on a five or seven day rotation will probably cost more than you really want to spend. The only thing I can tell you is that it is necessary, and cheaper than the cost of losing your data.
Let me explain by example.
Lets say that a fellow named Pete operates his own small business from home. He chooses to use an Iomega ZIP drive for his backups and determines that four 250MB disks are enough to do a full backup of his files. Since these disks cost around $17.00 each and he's under a tight budget, Pete decides that he will make do with only one backup set of disks.
Since he's only using enough disks to do one full backup, he doesn't have the space left to run any additional differential or incremental backups on the last disk. Pete does realize how important it is to backup his data every day, so this means that each day he will have to run a full backup in order to keep his backup files up to date. It's a hassle, but Pete figures it's worth it in order to save money at the same time as protecting his data.
Can you see where Pete's backup system would put his data at risk?
Each time Pete runs his backup, he's overwriting his one and only good backup copy of his files. As long as all goes well during the current backup, this system may serve its purpose for Pete. But it has the potential of disaster built into it.
Let's suppose one day Pete is doing his daily backup and something goes wrong, the files on the disks become corrupt. Pete doesn't know that he has just replaced his last good backup with garbage....he thinks his data is perfectly safe. Pete is totally unaware of the fact that he is standing on the edge of disaster.
If I may take the liberty to digress into a more philosophical frame of mind for a moment..... have you ever stopped to wonder how two random events can come together in your life with just the right time to create a significant impact on your life? This impact could be a positive one, or it could be negative. In Pete's case, I'm afraid it was negative. The first random event to enter Pete's life was the corrupted backup. But it would be no big deal and his life would go on happily, if only Pete could make it through until tomorrow's backup without incident. What are the odds that a corrupt backup would be followed by a hard disk crash the very next morning. Two random occurrences clashing in a point in time and space to form the shape of disaster....
When Pete turned his computer on the next morning, he was not happy to see the dreaded "no disk detected" error on his monitor screen. But he took comfort in the fact that at least he had his backup. Imagine his dismay when, after restoring his data, all he found was so much gobblygoo on his brand new hard drive.
Do you see the wisdom in using at least two backup sets now? If Pete would have used at least a two set rotation method, he would probably still have his data. Using a two set rotation, you would back up on one set of disks one day and then use the second set the next day. When you backup, you always backup to the oldest backup set (which would be from two days previous if you backup everyday). This way, you will always have yesterdays backup copy in case something goes wrong. Sure, if Pete had done this he would have lost one days worth of information, but that is a far cry better than losing all the data!
Techies would not approve of this method, I'm afraid, because you still only have one backup to rely on. The minimum rotation Techies recommend is a one week rotation (a week being either a 5 or 7 day work week). With this method, you mark each set of disks for the day of the week Monday through Friday. Obviously, you use the set marked for the particular day of the week you are backing up on. With this method, you will always have four or six days of backups (depending on the cycle week you choose) to draw from if you need to restore your data. Techies have other more elaborate rotation schemes as well, such as monthly, yearly, and the Grandfather-Father-Son Rotation. UUmmmm.....I think we'll just leave these methods for the Techies.
You can be fairly confident that with the weekly rotation method, your data will remain secure. Since the likeliness of losing all the tapes to corruption is extremely unlikely, the most data you'd ever lose is one week. Still more preferable than losing all your data.
Another thing you should do to make your data more secure is to always have your backup program verify the integrity of your backup data. Most backup programs have this option. It takes a bit longer to run the backup, but at least you know that the time you took was not wasted.
You should also consider doing a complete full backup on a monthly basis. You can use the rotation method for this as well. And a year end full archive backup would not hurt either.
In the end, it will be up to you to ensure your data is safely backed up. The number of backup sets, the backup type you perform, you will need to decide for yourself what will
work best for you. Just remember the statistics I gave you in the beginning, that 92% of businesses which lose their data and are unable to recover it, fails within the next year. Don't risk your
business data to the random elements of time and space, for pete's sake! 
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