Take a moment to reflect on some of the things that made you anxious when you were a child. One of these was probably the fear of getting separated from your mom or dad in a strange place like a crowded department store and never seeing them again. Another might be a fear of what kinds of creatures could be lurking in the darkness or the angst of being home alone and hearing spooky noises. Yet another could be the nervousness associated with the first day of school and hoping that you could fit in. The universal themes of isolation, vulnerability, dread of the unknown and assimilation are the cornerstone of all scary stories; the only elements that change are the settings, the characters and the objects of terror.
Anything can be scary at a cemetery, a boarded-up mansion, a creepy castle, a bayou swamp or deserted fairgrounds. These settings are used frequently in scary stories, novels and movies, and audiences never tire of them because they instantly conjure apprehensions that death is skulking around every dark corner. Their popular use as a backdrop is usually in connection with a dare (i.e., "Bet you $100 you can't survive a night in that creepy castle") or arises out of desperation (i.e., someone being chased realizes his only chance of escape is a shortcut through the graveyard). While there's nothing wrong with using these setups---which invariably transpire at night---experiment with daylight plots involving venues that readers visit often yet would probably dismiss as the platform for a fright fest. Examples: car washes, grocery stores, libraries, gyms, dry cleaners, video stores, banks. By using a setting that's familiar to them and spinning a scary premise, you're not only making them picture a real place as they read but also planting the seeds to remember your story every time they go get their cars washed.
One of the biggest mistakes that new writers make when spinning a tale involving bloodthirsty adversaries is that the mayhem they direct toward hapless humans has absolutely no rationale behind it. They are bad simply to be bad. As a result, many of the monsters, mummies and ax-wielding psychopaths in the woods come across as cartoon caricatures engaging in repetitious and predictable behaviors. While the objective of a scary story isn't for readers to root for the swamp thing's happiness, it makes for a stronger premise if the writer has provided some insight into the swamp thing's point of view. Ghost stories often do a good job in this regard because they establish early in the tale that the spirit (1) wants to avenge her murder or that of a family member, (2) wants to be reunited with her one true love or (3) wants to be left alone. Whatever form your story's "bad guy" takes, the actions need to be driven by reasons that are just as compelling as those of the protagonist. Likewise, the protagonist of a scary story has to have better reasons than just a dare or boredom to go dig up a coffin, dabble in the occult or disobey signs that warn "Enter at your own risk."
There's only so much scariness you can inflict on readers or listeners before they grab their heads and say, "Enough already!" While it's critical to keep escalating the danger and suspense at every turn, you can maximize your scariest scenes and revelations by either using a comic relief character or giving your audience a slight breather with something funny and nonthreatening. The comic relief character is often the likable joker of the group; your target audience relaxes a bit whenever this guy is around because nothing bad ever seems to happen while he's making light of the situation and being the group cheerleader. Your readers are then lulled into such a comfort level around him that they'll be shocked when he gets gruesomely dispatched by the swamp thing. As for inserting a humorous breather, the most common scenario is when the lead character approaches a cobweb-laced spooky door and starts to push it open. The tension is thick because readers just know that the monster is ready to pounce out. The door opens to either reveal an empty room or a cat that's grateful someone finally came to rescue her. Just as the reader breathes a sigh of relief and perhaps even smiles that the cat poses no danger, the monster grabs the hero from behind. Gotcha!