After exclamation points and profanity, italics are the writer's third greatest enemy.
Like profanity, italics are most effective when used sparingly. From my point of view, italics should only be used to:
- place emphasis on a particular word or phrase, such as a foreign word or phrase;
- cite the name of a book, film, ship, television or radio program, or musical work (as in the name of a symphony or a specific album, such as Mahler's 1st, The Who's Tommy, Warren Zevon's Bad Luck Streak In Dancing School, etc. ... see A Guide to Italics, Quotes, and Emphasis for many more examples);
- to insert a brief flashback - be it a sequence of events or a snippet of recalled conversation - within the body of the current narrative; and,
- to set apart the contents of a letter, excerpted lines from a poem, or a snippet of song lyrics (which could arguably be accomplished with the use of block quotes instead, making this last "rule" more of a stylistic choice on the part of the writer).
(Parenthetical pause here: when citing the name of a song or a story, quotation marks are what's required, as in: Simon and Garfunkel's "Bridge Over Troubled Water" or Stephen King's "Sometimes They Come Back". The title of the album or collection in which the piece is included would be italicized, as in: Simon and Garnfunkel's Greatest Hits and Night Shift. The differences are subtle, and not necessarily as easy to discern as one might at first think.)
Remember the Dragnet-theme warning I suggested when it came to using exclamation points? (Quick recap: imagine that every time you use an exclamation point outside of dialogue, it comes accompanied by the first four notes of the Dragnet theme; you'll use them sparingly as a result.)
Well I've got a similar warning cue to employ when it comes to italics: imagine that whatever is italicized is being either whispered or Shouted Through A Bullhorn (however circumstances dictate); it's a matter of extremes, like it or not.
An italicized letter or quoted poem? A whisper.
A panicked warning (as in: "Look out!")? A shout through a bullhorn. (And bear in mind that when you combine italics with all caps -- "LOOK OUT!" -- it's overkill; the circumstances under which something like the above is italicized give the words or passage an immediacy that presenting them in all capital letters only diminishes; it's hitting the reader over the head with your intent: DEAR GOD, THIS IS REALLY, REALLY IMPORTANT AND I'M GOING TO MAKE DAMN SURE YOU KNOW IT!. Overkill. Don't do that.)
There is another -- and less directly acknowledged -- reason that it's a good idea to use italics sparingly: like it or not, a prolonged passage of italics quickly tires the eyes while reading. It's that simple.
As a writer, whenever I come to a passage that I know is going to have to be italicized (such as a letter or brief flashback), I apply the same rule to my own work that I do to anything that I might choose to read: no more than 3 pages. That is all that my eyes can take as a reader, so I assume that's my readers' limits, as well. After 3 pages, it just gets annoying; and the last thing you want is for a reader to become more aware of how you're presenting something than of its content.
So: a whisper or shouted through a bullhorn, no more than 3 pages, and you just might find that italics can be a useful ally, rather than the enemy.