Compared to the guidelines for the preceding punctuation marks, the rules for the dash are a little more freewheeling. However, be careful not to overuse it.
3.4.1 Parenthetical Expressions
Use a dash to set off a parenthetical expression when the dash is necessary for the sake of clarity.
I called my mother, a former principal, and my sister.
This sentence is confusing. The mother is the former principal alluded to, yet because of the punctuation it may seem that the writer has in fact called three people: Mom, a former principal, and a sister. A dash can fix this problem:
I called my mother — a former principal — and my sister.
In the following example, too, the dash clears up potential confusion:
We invited CEOs of three major firms, General Electric, Goldman Sachs, and Viacom, to the conference.
We invited CEOs of three major firms — General Electric, Goldman Sachs, and Viacom — to the conference.
3.4.2 Dramatic Emphasis
The dash can also be used for dramatic emphasis in various contexts. For example:
The fire destroyed their house, their car, and all of their belongings — but it could not destroy their hope for the future.
Sometimes it can be difficult to choose between a colon and a dash. Where a colon is suitable, you should generally use that rather than the dash. For example, in the following sentence a dash might work, but the sentence structure is ideally suited to the colon:
Joe has three ambitions — a good job, a nice home, and a dozen children.
Joe has three ambitions: a good job, a nice home, and a dozen children.