From the rather profound The Tao of Backup, with a few typos fixed and a little extra emphasis:

The novice asked the backup master: "How often should I back up my files? It has been a month since my last backup."

The master replied: "Just as night follows day, and Autumn follows Summer, so should backups follow work. As you work, so should you back up that work."

The novice said: "I work each day."

The master replied: "Then you should back up each day."

The novice replied: "I agree, but right now I haven't got time to make a backup, as I have too much work to do."

Upon hearing this, the master fell silent.
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Having defined the data to be backed up, you should perform backups in accordance with the rate at which you are creating data to be backed up.

The frequency with which you back up should be determined by

* The rate at which you are creating data.
* The cost of losing data.
* The cost of performing a backup.

It's possible to create a mathematical model to formalize all these factors and determine an "optimal" backup strategy. However, such models will not usually reflect the true human cost of losing data. For example, if someone writes a poem or an essay, having to rewrite the poem or essay will usually be stressful to the writer, and the result may not be as special as the original. Similarly, if a potential new customer phones you, and you enter their details into a contact database, and then lose the database, there may be no way of contacting that customer ever again.
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All these considerations mean that the best "strategy" is to back up often enough so that if you lose a random disk at a random time, the chances are that you will lose very little data indeed. This usually means daily backups, combined with disk-to-disk copies or mirroring during the day.

Send this to all your friends, with the subject line "Guess what lucky duck is getting a fantastic present from me today!" Even they get mad, they'll still have subconsciously absorbed the words "backup" and "daily."

Here's a story I've been retelling for years that has suddenly taken on even more poignant meaning. From Adam Kirsch's ArtForum review of James Laughlin's memoir Byways:

In the early years especially, Laughlin's authors were sometimes forced to cool their heels and gnash their teeth while their publisher spent time at his ski resort in Alta, Utah. (There is a painful stretch in his published correspondence with Delmore Schwartz involving a lost manuscript that finally turned up under the floorboards of an Alta mail truck.)

As I remember it, before the manuscript was rediscovered Schwartz buckled down and rewrote it and, of course—here writer-listeners shudder like campers unable to resist nightmare-baiting ghost stories—it was better than ever! First, though, he had a crippling breakdown. Do try to avoid that, won't you?