Agatha Christie may have been a pioneer in the art of the mystery, but you're not likely to succeed today if you copy her plotting techniques and handling of clues. To emphasize the brilliance of her sleuths, Christie often kept the reader in the dark about what the protagonist knew, and she didn't hesitate to solve the crime by introducing a new character or element at the end of a story.

Today's mystery fans expect writers to play fair. Readers, and therefore agents and editors, will no longer accept the sudden appearance at the climax of a villain who's never been seen or mentioned before. They want the killer -- masquerading as an innocent party -- to show up early in the book and appear at least a few times during the story. Readers want to know what the protagonist knows, and they expect the writer to provide enough clues to allow readers to either solve the crime along with the sleuth or look back over the story and say, "Ah, now I see what this and this and this meant."

A clue is anything that points to the killer's identity. Clues come in many guises and, far from making a story formulaic, they can be used to create original mysteries that keep readers enthralled to the end.

In addition to genuine clues, you need red herrings, false clues, and misdirection. Red herrings -- the term comes from the practice of dragging a dead fish across a trail to throw hunting dogs off the scent of their prey -- can be factual, but they take the sleuth (and reader) in the wrong direction and don't help solve the crime. In Tess Gerritsen's The Sinner, the discovery that the murdered nun recently gave birth appears at first to be a vital clue to the crime, but it's a red herring that has nothing to do with the killing.

False clues can be planted by the killer to make an innocent person look guilty and prevent the police from finding the real culprit. "Evidence" that the murder victim was raped in Scott Turow's Presumed Innocent is a false clue. Misdirection can be accomplished by burying a clue in a scene, letting it come from an untrustworthy source, or distracting the sleuth immediately after the clue is revealed, so that its meaning is overlooked. Placing a clue in plain sight can make it so obvious that both detective and reader will dismiss it.

The genuine clues you might use include:

• Forensic evidence at crime scenes -- blood, fibers, hairs, clothing, "signature" objects left by the killer, etc.
  
  
• An unusual murder method that points to the killer's profession or emotional state.
  
  
• Items in the victim's or suspects' homes -- letters, jewelry, photos, almost anything that can reveal a relationship between the victim and the killer. As with crime scenes, something that should be present might be missing, or something may be found that shouldn't be there.
  
  
• Emotional ties to the victim -- love, hate, greed, and jealousy can all lead to murder.
  
  
• Secrets -- everyone has secrets, and so must your characters. A murder investigation exposes the secrets of the innocent as well as the guilty, and the need to hide information makes people behave furtively, self-defensively. One secret will lead to the killer. The rest will entertain the reader and confuse the sleuth.

Deciding who your murderer is and outlining the plot before you start writing simplifies the clue-planting process enormously. If you prefer not to outline, you may still produce an excellent book, but you might have to do some backtracking and rearranging of plot elements to accommodate your clues.

Philip Margolin, speaking on a panel at the 2005 Mystery Writers of America symposium, said that he outlines every book exhaustively, scene by scene, working backward from the solution. He plants clues and red herrings as he outlines, spacing them out so they won't tip off the reader too early.

Jacqueline Winspear, on the same panel, said she also outlines, but her clues tend to be organic, tailored to the special abilities of her character, Maisie Dobbs. She has to be careful, Winspear admitted, not to leave things out. She rereads her manuscripts "as an advocate for the reader" to determine whether she's actually transferred all of Maisie's perceptions to the page. She tries not to let the reader spot things that Maisie initially misses or doesn't understand, because that would make the protagonist look stupid.

Kristy Montee, who writes as P.J. Parrish in collaboration with her sister Kelly, said during the MWA panel that she sees clues as a "bread crumb trail" leading to the killer, and because her books are procedurals they need a lot of those crumbs. However, she lets the story take its course, then goes back and deliberately plants clues where needed. She doesn't revise as she goes -- "If you rewrite a book constantly, you'll never finish." She does, however, make liberal use of Post-It notes to remind herself where she needs to place a clue, a red herring, or a bit of misdirection.

Margolin said he likes surprises, the more the better, but warned against exhausting the reader by trying to produce a new twist on every page. If you can fool the readers once -- "Make them think they've figured out exactly what's going on, then pull the rug out from under them" -- they won't trust their own instincts after that and surprising them will be easy.

Twist Phelan, the panel moderator, noted that a surprise ending depends on the reader overlooking or failing to understand clues. One final clue ties everything together and reveals the killer. Too many writers, Montee said, try to pull off a big twist at the end when all they really have to do is solve the crime.

Margolin agreed that the ending is often the weakest part of a mystery. This can be due to the grueling book-a-year deadlines mystery writers must meet. They rush the writing, fail to place clues properly, and produce endings that seem abrupt and may not make a lot of sense. Unpublished writers don't have that excuse, Margolin said. If you're not yet bound by a deadline, take your time, think your book through to the end, and be sure your plot, your clues, and your ending work logically. Then some publisher will sign you to a contract that calls for a book a year, and soon you'll have strict deadlines of your own to worry about.