However--and this is a big however--far too often flashbacks are completely overused. Some novels, like John Carroll's The Ghost in Love, include around a hundred little flashbacks to let readers in on tiny background details. These details add little to the actual conversation, but merely seem to make the characters sound more quirky, or interrupt the action at hand.
And then, of course, television offers a bunch of these sorts of things, too. Lost and other shows sometimes spend whole episodes in one character's past, and these episodes risk losing an entire audience who really wanted to keep following the current story.
I tend to dislike rules in general, but I do keep a few things in mind when I use a flashback, even a brief one:
1. The flashback should illustrate the bigger story, not take it over.
2. Clarity is crucial, for confusion can lead readers to stop reading.
3. A character's history is not nearly as important as what he is doing now.
I create tons of history for all of my characters--past experiences, old relationships, etc.--but I tend in my own life to be caught up in the present, so I like my novel reading to be the same. That is why I thought The Ghost in Love was a pretty lame novel (boring and lame). It's also why I don't watch Lost (not since season 2).
Still, flashbacks have a place in writing, even in my own writing. Knowing the past of a character can help readers understand them more in the present. I just use this technique sparingly, to minimize the distraction.
How do all of you use flashbacks? What purpose do they serve in your own writing?