So I thought it'd be interesting to put together a list of major elements for creating tension, and see what the rest of you might have to add to it. :)
Disclaimer: I mostly read speculative fiction, so most of the titles I reference here are going to be from that genre. I think most/all of the points made here apply to contemporary, realistic fiction as well. I am only using book titles in the examples; if a title isn't familiar to you, there's a list of all the books referenced with authors at the bottom of the post. Also, note that no matter how much tension you stick in a story, it generally isn't going to work unless you have created characters the readers will care about in the first place... which is a whole 'nother topic.
And, of course, these are only my theories and you are welcome to disagree.
I'm going to assume we all have a pretty clear understanding of what tension is and how to introduce it. As I see it, tension is about suspense and uncertainty--the reader doesn't know what is going to happen next, or how the story is going to get to the point they think it's heading toward. And tension is important because as soon as the reader thinks they know everything that's going to happen, they're likely to get bored and lose interest in reading on. (After all, why bother to keep reading if you think you already know the story?) So as writers we put our characters in sticky situations and throw obstacles between them and their goals, and hope this is enough to keep our readers engaged.
But what if you want to write one of those 'oh wow I only meant to read the first chapter and I ended up staying up until three in the morning just to find out how it ended' type books? Possibly you need a little more. Here's a few things, alone or in combination, I think can kick a book from "that was nice" to "omigod I could not put this down."
Creating Tension Via Plot
Threaten someone's life. Threatening to take away something a character cares about is always a great tension-builder, and it's most potent when that something is a person they care about (or themselves) and the loss would be permanent. Fear of having a loved one die (or dying ourselves) is something just about any reader can relate to. The threat is most intense when it's immediate: in THE HUNGER GAMES, Katniss is under attack and fighting for her life from the second the games begin; in THE CHANGEOVER, Laura can see her brother is sickening so fast he won't last the week. But it can also be incredibly gripping when it's a drawn-out struggle against death, as in LIFE AS WE KNEW IT, where every day is a victory against starvation and the cold. It's hard to put down a book when the characters are walking the line between life and death, and you're not sure if they'll make it.
Threaten a whole bunch of people's lives. This is not quite the same as the first point, because when it's a bunch of people threatened, the threat is usually less personal to the main character (although there are books, like TOMORROW WHEN THE WAR BEGAN, that do an excellent job of making the big threat personal--it's not just "our town" that's under attack, but the characters' parents, siblings, and friends). But the possible lack of personal stakes are balanced out by the size of the threat--no one wants to see a massacre. It can be a few "bad" people menacing a small group (see THE GOLDEN COMPASS--which increases its stakes by making the people threatened a) kids and b) a few of them kids the main character knows), an environmental crisis (in THE CITY OF EMBER, the biggest threat to the city is simply running out of supplies), or large-scale battle (ever notice how the tension doubles in THE GOOSE GIRL the moment Ani finds out the two countries she's torn between may go to war?). Whatever the case, setting aside a character who's struggling to save the lives of dozens, hundreds, or thousands of people is rather difficult.
Set the clock. Bad enough if someone you love--or your whole city--is about to be destroyed. How much worse if you've only got a day to set things right? The time limit can be exact (in THE GOOSE GIRL, Ani must reveal her rival's deceptions before the wedding takes place; in THE WITCHES, if the boy and his grandmother must defeat the witches before they leave the hotel the next morning) or vague (in THE CITY OF EMBER, we know the city's resources are running out, but we don't know exactly how long the characters have to find a way out), and both have their advantages. A specific time means the reader is aware of exactly how little remains; a vague time means it could run out at any second without warning. Time-tension can also be done in intervals rather than one big lead-up--take THE HUNGER GAMES, where every hour that passes without a death is an hour closer to some new threat or rule that will be thrown into the game. Putting a time limit on how long the characters have to solve their problems (whether they're matters of life and death or somewhat more mundane, like getting that extra credit before college apps go in) makes the problems that much more urgent, and the tension that much sharper.
Stick them between a rock and a hard place. A choice between an option a character likes and one s/he doesn't is really no choice at all. The most tension comes when there is no "good" choice, when all the available options mean sacrifice and it's not clear which would have the best outcome. The choice and the sacrifices can be personal (in BOOK OF A THOUSAND DAYS Dashti has to choose between honesty and her loyalty to her lady), or broader (in NORTHLANDER Ellin has to decide whether to support the uncertain friends who would stir up a war and the equally uncertain friends who have created the grounds for war in the first place). Either way, when watching such a choice play out, it's easy to become glued to the page wanting to see what the character will decide and what the consequences will be.
Next time, punch harder. Like most things, a source of tension, if it stays the same, starts to lose its impact over time. But you don't always want to throw some completely new obstacle or villain into your characters' way. Instead, just before the reader has time to recover from the last set-back, hit them a little harder. Each time things get worse, you're setting up a standard that says, no matter how bad it seems now, there's something even more awful around the corner. Whether it's a villain who becomes increasingly hostile (like Cezar in WILDWOOD DANCING, who starts with questionable remarks and moves on to taking over the protagonists' finances, home, and finally their freedom) or a situation spiraling gradually downhill (as, in LIFE AS WE KNEW IT, the environmental catastrophe forces the characters to move from the entire house, to the first floor, to the only livable room left), that sense of 'how can this possibly get any worse?' and the knowledge that the writer has proved that it can more than once keeps readers on their toes--and reading.
Creating Tension Via Characters
Keep them in the dark. Whether it's a narrator who doesn't know the whole story or one who's deliberately hiding information from the reader, what isn't said can create a lot more tension than what is. Offer a narrator who's only in a position to see part of the big picture (like Ellie in TOMORROW WHEN THE WAR BEGAN, who stumbles back into her war-torn town with no idea what's happened or why, or Costis in THE KING OF ATTOLIA, who only sees a fragment of the king and court's schemes), and readers can't help but wonder how much there is they don't know--and when it'll pop up to surprise them. Offer a narrator who knows far more than s/he's telling (like Nick in BREATHING UNDERWATER, who refuses to address exactly why his ex-girlfriend had to take him to court until he's told the story from the beginning, or Parker in CRACKED UP TO BE, who skirts around the thought of the night that changed everything, revealing only a little more detail each time), and readers will be combing each page for clues as they read on, eager to put the pieces together.
Point out their mistakes. Even a character who thinks they know all there is to know about a situation... probably doesn't. The more you reveal that they've misjudged or been mistaken, the more uncertainty (and so tension) there is about all the "facts" that haven't been challenged yet. When Charlotte (in THE TRUE CONFESSIONS OF CHARLOTTE DOYLE) finds out the crew isn't as awful as she expected, or when Sophie (in HOWL'S MOVING CASTLE) discovers that the "evil" wizard Howl hardly lives up to his terrifying reputation, the reader knows more revelations are on the way--and wants to be there to see them.
Paint them shades of gray. Generally speaking, if you find a book where the heroes are clearly good, and the villains clearly evil, you go into it feeling fairly certain of the outcome--good will prevail. (Unless you have reason to expect this is a really depressing book.) But what about when the heroes aren't right about everything, and some of the points the villain raises are actually kind of reasonable? Suddenly you have tension not just over how good will prevail, but who is good, exactly--who should we really be rooting for? Take COMPANIONS OF THE NIGHT, where Kerry is never completely sure that Ethan is any less dangerous than the man who kidnapped her family, or PRIDE AND PREJUDICE, in which Elizabeth is faced with conflicting stories about her main suitors and their treatment of various sisters. As long as readers aren't sure who is right and wrong (or more right and more wrong), the outcome stays unpredictable and the desire to discover it undiminished.
Jerk the rug out from under them. All characters have some sort of support system: people, places, and things they rely on to help them through a crises. So, naturally, if you take away that support system, their chances of making it through the challenges that face them (in decent shape) go straight down--and the tension skyrockets. Plunk them down on their own in a completely unfamiliar environment (as happens to Charlotte in THE TRUE CONFESSIONS OF CHARLOTTE DOYLE and Hahp in SKIN HUNGER) and watch them scramble. Or strip them down bit-by-bit (in THE GOOSE GIRL, Ani loses her country, her position, and her best friend, one after the other) and let readers wonder just how much more they can take. A character off-balance in the midst of a struggle can be far more exciting to watch than a character who stands on firm ground.
Creating Tension Via Technique/Structure
Take their breath away. When you give the reader a chance to breathe and say 'okay, things aren't so bad right now, I'm sure everything will be okay', you also give them a moment where they can, without much discomfort, set aside the book and find something else to do. This tends to happen at chapter ends, which is why you so often see the advice to end your chapters with cliffhangers. But I think it's a technique that can be applied more widely. Simply never resolve all of your character's immediate conflicts. Rather than introduce one conflict after the last one is resolved, stagger them. If the characters are about to find the answer to one mystery, throw another in their path just before. If they're about to get around one obstacle, show the next one just over the horizon. If they never get a chance to stop fighting, the tension never has a chance to ebb. (The most striking example of this I've seen is REBECCA, from around the midway point on. I meant to stop reading for the night as soon as the most immediate issue facing the main character was resolved. But before it was dealt with, another, even more pressing, issue popped up. And another. And another. And before I knew it I'd read all the way to the end because there just wasn't any place I could let those characters go without their predicament haunting me while I tried to sleep.)
I recommend you use this in moderation, because of course if you make your readers hold their breaths too long, they're just going to get dizzy and stop reading. It can also be done in a more subtle, but still very tension-inducing way, by having the character's entire situation be one immediate problem (as in HOUSE OF STAIRS, where you are constantly aware that the characters are trapped and want to escape, or LIFE AS WE KNOW IT, where the effects of the disaster are ever-present and impossible to ignore).
Offer a sneak peek. You know this sort of framing device--start off the story with a tense scene from the middle or end of the book, and then go back to the beginning to tell the story in order. You're letting readers know something terrible is coming--and now they'll spent the rest of the book, even when the characters are at their happiest, wondering how on earth it's going to come about (and, of course, how the characters will get out of it when it does). I remember this being done to excellent effect (at least to me as a teen reader--it's been a while since I read it) in LOSING JOE'S PLACE. BREATHING UNDERWATER starts with the main character getting a restraining order and leaves readers wondering up until the end just how bad things got for his girlfriend to think that necessary. And a flashback is not always necessary for this to work--simply having a narrator who's relating events after the fact can add an ominous tone to otherwise straight-forward proceedings. From the very start of TOMORROW WHEN THE WAR BEGAN, it's clear something horrible has happened that the characters feel the need to document, and it's impossible to forget that even during the playful adventure that starts their story.
Play mean. In all sorts of fiction, but particularly children's and YA (because there seems to be more of an aversion to unhappy endings there), there's a danger of complacency--of the reader feeling that, no matter what happens, they can be sure that the main characters will survive and come out of the book's events relatively happy. Unfortunately, complacency is an enemy of tension. Think of the suspense when you really, truly aren't sure whether you'll get the happy ending you're longing for. How much more important it is to reach the ending of such a book--because you're not certain what sort of ending it'll be.
That sort of uncertainty, from what I've seen, is mostly established by playing mean instead of nice. Letting horrible things happen to the main characters. Letting them make awful mistakes. Letting them or their loved ones die (for real, not 'just until we reveal they didn't really die after all'). Letting the villain win, some of the time. I think this is a large part of what made the Harry Potter series so gripping for so many people. Important characters died and didn't come back. Harry and his friends and supporters screwed up, rather a lot at times. The heroes won minor victories, but with each book the villains gained more and more ground. And because of this, readers weren't sure how happy the ending would be--I saw many people speculating that Harry would die, and/or other major characters. (In fact, I think some people were a little disappointed that more major figures didn't die in the last book.) If every book had finished with Voldemort no closer to taking over the wizarding world than he had been before, and everyone Harry cared about alive and well, would there have been quite the same frenzy to get and read the next book? I suspect not.
(For other examples of 'playing mean' in YA books, see HOUSE OF STAIRS and CRACKED UP TO BE, both of which take the characters to their very lowest points.)
PRIDE AND PREJUDICE by Jane Austen
THE TRUE CONFESSIONS OF CHARLOTTE DOYLE by Avi
NORTHLANDER by Meg Burden
THE HUNGER GAMES by Suzanne Collins
THE WITCHES by Roald Dahl
SKIN HUNGER by Kathleen Duey
REBECCA by Daphne du Maurier
THE CITY OF EMBER by Jeanne Duprau
BREATHING UNDERWATER by Alex Flinn
HOWL'S MOVING CASTLE by Diana Wynne Jones
BOOK OF A THOUSAND DAYS by Shannon Hale
THE GOOSE GIRL by Shannon Hale
LOSING JOE'S PLACE by Gordon Korman
THE CHANGEOVER by Margaret Mahy
WILDWOOD DANCING by Juliet Marillier
TOMORROW WHEN THE WAR BEGAN by John Marsden
LIFE AS WE KNEW IT by Susan Beth Pfeffer
THE GOLDEN COMPASS by Philip Pullman
THE HARRY POTTER SERIES by J.K. Rowling
HOUSE OF STAIRS by William Sleator
CRACKED UP TO BE by Courtney Summers
THE KING OF ATTOLIA by Megan Whalen Turner
COMPANIONS OF THE NIGHT by Vivian Vande Velde
So, that's my list. What would you add to it?
What are your favorite ways of increasing the tension in your stories?
What types of tension do you find affect you the most when you're reading?