Keep It Short
Noir short stories and novels are almost always written in a concise, raw style that does not involve a great deal of explaining or wordiness. Stark and barren landscapes, empty cities at night, or decrepit warehouses can be often-used backdrops for the story. Tell the reader enough to know where they are, but be spare with the language. Noir fiction characters don't tend to have much emotional depth. They scheme, they strategize, but they aren't blabbermouths. Keep it simple, and focus on plot (and plot twists), mood and pithy dialogue.
The Criminal Is the Star
It's conceivable that a noir story can have a detective as the main character, but noir fiction tends to revolve around the criminal. It can be a man or a woman, but in a traditional noir story, there absolutely needs to be a femme fatale. Debates have raged about whether this archetype is sexist or a sign of admiration of females, but whatever the case, noir stories need a tough-talking and unforgettable woman. Show us what drives the male or female lead. How has he gotten to this desperate point? What drives her to a life a crime? Noir stories portray a criminal who is seeking one last chance at something big: a heist, a scam or a murder.
Remember the Shadows
Plot points might be hatched in an outline before you get started writing, but don't forget about the mood and atmosphere. Plan for this, as it is important in noir. Don't think of shadows, dark cities and smoke-filled rooms as hackneyed cliches. In noir, they are necessary. If you're writing something more contemporary than your average 1940s-style noir pulp, you can dispense with the trench coats and the rumpled suits, but keep the setting consistent. If you want desolation, set the story in a desert town in Arizona. If you need skyscrapers, set it in a city. The setting should in every case be as distinct and as bleak or seedy as the story itself.
No Happy Endings Here
The plot of any noir fiction revolves around a crime, and likely a relationship between two desperate people, as well. Whatever your plot is, a typical undercurrent in the story is a pattern of self-destructive behavior on the part of the main character. Desperation is usually offered up as the explanation for this phenomenon, but dig deeper into your own psyche to figure out what drives this individual to strive for eventual ruin. A noir character may say, "I was just born bad. I can't help it." You as the writer know more. You're the creator, and have the absolute power to give your antihero whatever past or whatever failing you desire.