Become a ghost hunter: If you're going to write a scary story, you might as well experience some scary stuff (this doesn't include the ‘house of horrors' in your local amusement park). So pick up those Geiger counters, ghost traps and night vision cameras and head to the nearest deserted house with an anomalous reputation for being spooky. Yes, you have to go there only at in the night time. No, you can't take anyone along. The key to being a good writer is to experience things in isolation. Even if it's the last thing you will ever experience.
Research: To write a good scary story, you need to know well the ways of an apparition or ghost. Do some research, ask some people who have experienced the paranormal. Put in your own experiences and read some good horror stories. Stephen King is good. “The Exorcist” or “The Omen” is better. “Twilight”? Nah, that isn't really horror.
Good catchline: The best part of any horror novel is a catchline that's really creepy. Don't use clichés such as like ‘be afraid' or ‘it will leave you screaming' since you're can't be sure exactly why the book will leave the reader screaming. Instead, something on the lines of wordplay. Maybe ‘she looked in the mirror one last time' or something just as simple as ‘3 a.m.' will ould do. It just has to get the reader curious — Adding a bit of oozing goo on the cover or a scary face with only the white of the its eyes showing turned up isn't scary. It might just turn out to be hilarious.
The twins in the hallway: If you've ever watched “The Shining” (Stanley Kubrick's version of Stephen King's novel), you'd realise how scary the infamous ‘twins in the hallway' scene is. Try to imbibe some lessons from this scene, and infuse it in your book. They don't have to be twins who seem dead and... well, undead. You can get creative and maybe kill your protagonist, before bringing him back to life again. And, kill him again. And, bring him back. And… Oh, you know the drill!
No good ghosts: Always understand that a ghost in a horror novel has to be a bad ghost. He/she has to be scary, with a violent death and an unfinished business. But under no circumstances, pre- or post-climax must the evil turn good. No Casper-the-friendly-ghost's-long-lost-nauseatingly-happy cousin who is a do-gooder. Your ghost must destroy all good. He must appear in dark hallways, shift shapes into glowing orbs, photograph as glowing mist, stare sullenly from attic windows and scare the creepers out of anyone. Even you.