“I would’ve gotten away with it if it wasn’t for you meddling kids.”

10. (extension of 8- your denouement) Keep something out in the open hidden for as long as possible Or, Create really convenient coincidences.

It just so happens before the great detective leaves or before the case is left open, he stumbles across something that never manages to reveal itself until the very last possible second. For instance, in an episode of Monk, before his vacation departure, he looks at pictures from his trip and realizes there were three suitcases on display in the lobby in the beginning. That could only mean one thing, that new fourth piece of luggage is also where the killer stashed the victim’s body.

11. (extension of 7- the climax) Place your detective in some kind of peril.

I think what lacks in the Poe mysteries (Rue Morgue, Purloined Letter) is he doesn’t have enough tension when he’s got the person connected to the crime. They’re great stories but, the conclusion is handed to you on a silver platter- no trouble necessary. Dupin’s got the culprits figured out and anticipates their moves before they have time to even act. So I say endanger your hero, they’ll live anyway. CSI Detective Nick Stokes was buried alive and over a ticking time bomb, Veronica Mars was almost burned alive and they survived.

12. Give your characters an unforgettable catch phrase.

“It’s elementary Watson.” (Sherlock Holmes)
“I would’ve gotten away with it if it wasn’t for you meddling kids.” (Scooby Doo)