This has been on my mind lately, since pretty much all of my writing is suspenseful. There's such a fine line between making a scene or a moment terrifying or just... cringe-worthy. So here are some methods we can all use to make our stories truly horrifying (note word choice here, hee-hee).
1. Make sure your main character is running around in a skimpy nightshirt.
Seriously, this is critical. I don't care where your main character is or what she's doing, whether she's bolting from bed or a busy restaurant. She's got to be wearing a near-transparent, so-short-we-can-see-pretty-much-everything t-shirt. She can't be wearing shoes, either. It's just a given. I don't care if this has been done a million times. It's the only way your chase scenes will be good. It's classic.
2. Don't allow your character to make any intelligent decisions.
You know the term, "T.S.T.L."? Too stupid to live? No such thing! So what if your main character decides to sneeze during that hushed instant the killer is listening for her? So what if she decides to run down the middle of the road, in plain sight? Who cares if she doesn't call the police, for some freaking reason? In horror books and movies, it's expected that our main characters will be in such a panic there's no chance for her to use her brain. Why don't you just throw her in front of a bus, for good measure? That will really throw your reader off.
3. The ending shouldn't make any sense at all.
Say you're writing a murder mystery. You've filled the plot with twists and turns, you've taken us on this merry chase and it's finally come down to the big moment. The huge reveal. And then... it turns out a unicorn has been killing everyone. How great is that ending? No, you can't make the villain some guy we've been eyeing, who's a logical choice. You can't even make it someone mildly unexpected. It has to be just stunning. Forget unicorns. It's Frankenstein, come alive from the pages of his book! No, it's a mummy. No, no. It's you.
Whoa. Now there's a twist.