You can occasionally use a dash to separate part of a sentence and draw attention to it, just as you would with a pause in speech. Because dashes are versatile, it helps to know their three functions: linking an elaboration, setting off parenthetical material, or injecting a pause. Each use adds more emphasis than the standard comma, parentheses, or word space.

Some dashes link an elaboration, replacing a comma or a colon. Here's an example from Maureen Dowd in the New York Times:

But Saint Obama played the politics of character to an absurd extent. For 14 months, his argument for leading the world has been himself--his exquisitely globalized self.

With a comma instead of the dash, the passage would read:

But Saint Obama played the politics of character to an absurd extent. For 14 months, his argument for leading the world has been himself, his exquisitely globalized self.

The comma throws the reader too quickly into the trailing elaboration.

Elaborative dashes often attract more attention to a phrase than colons, as in this example from The Economist:

He is also interested in what some religions hold out as the ultimate reward for good behaviour--life after death.

Other dashes are parenthetical, helping readers wade through background or explanatory material in the middle of a sentence. Consider this example from The Atlantic:

The expensive cars [paparazzi] drive reflect the fact that Britney Spears--her marriages, custody battles, fights with her mom, new boyfriends, Starbucks runs, trips to the hospital--is a bigger and more lucrative story than Elizabeth Taylor and Richard Burton or John Lennon and Yoko Ono.

The dashes help readers process the details without losing the writer's thrust. The dash-less alternative is a jumble that makes it tough for readers to get from the subject (Britney Spears) to the verb (is):

The expensive cars [paparazzi] drive reflect the fact that Britney Spears, including her marriages, custody battles, fights with her mom, new boyfriends, Starbucks runs, and trips to the hospital, is a bigger and more lucrative story than Elizabeth Taylor and Richard Burton or John Lennon and Yoko Ono.

And still other dashes inject a pause, forcing a moment of reflection on what precedes them--before flinging readers into what follows. Here's an example from a World Bank World Development Report:

Yet billions of people still live in the darkness of poverty--unnecessarily.

Here the dash simply replaces a word space.

So when should you use a dash? Remember that dashes are pauses, so use them for emphasis--and sparingly. Also, avoid using them in a sentence that has a colon.