Ghost is the legend of horror movie, do not separate them.
I think that this is the first rule of thumb. There are many style of horror movie that made from different plot stories. A lot of horror movie producer may try to produce many style of movie, sometime they do not have ghost in their movie. If they miss this rules you are able to know exactly what is going to happened throughout the movie and this is kind of boring. But if they have ghosts in the movie watcher will always have questions about the movie and this is the critical point to judge whether the movie fail or success. The good example of this kind of movie in my opinion are The Blair Witch Project and The Sixth Sense.
Simplicity is the king.
I have seen many of horror movie try to create the story plot to be very mysterious and complicated by use many of characters and tricks. I’d like to tell you that the selling point and main objective of horror movie is to make people fun by being terrified not to get them deep down to very complicated quiz. So the good horror movie should stick with that rule and should focus on special effects and plot of story. However, effects, sometimes less is more when it comes to the horror movie there should not be loads of special effect to make people feel terrified but they can use sounds and visualization to make the movie exciting and scary.
Gory or Scary : Don’t be confused
Some movies has nothing but only bloodbath scene, full of guts spread around, vomiting and tons of decapitation. Wait wait wait, don’t be confused. If you think that these thing will make you feel terrified, you are completely wrong. These bloody scenes do not generate fear, they do generate disgust. If you are looking for a good horror to make you feel fear and scare, make sure that you do not choose the movie that has loads of these disgusting scene.
The mystery of costume.
This is quite difficult and not practical to explain and you may think that it may not important.
It may a bit hard for you know which movie you which style of costume for characters, but one thing is for sure that using flashy and dirty ragged costume, especially for ghosts may help on inspire fear but should not be overwhelm by them.