This is a great short video from TED-Ed / Victoria Smith on how to make your writing suspenseful.
Although this will not provide anything new to some writers, but it may at least act as a good refresh on the subject.
The scary moments are not when the monster jumps out, but just before that. — The suspense lies in the unknown, and the foreboding danger that lurks just around the corner.
The points below are summarised from the video:
Limit the POV: Instead of an omnipresent narrator – keep to 1st person or 3rd person limited, where the story is only told through the eyes of one individual. So this individual cannot see everything that is going on and hence this adds to the suspense.
Pick your settings wisely: Deserted buildings and old mansions work great. But can be a little cliched, so do set your scenes in such places, but try to add a spin or something original to it. Also adding bad weather is another characteristic of suspense and horror writing. But apart from setting the scene, it can also limit the visibility of the main or side characters, so something in the distance or nearby cannot be seen. – Thus adding to the suspense of the story.
Style and form: Build suspense by how something is conveyed and how it is paced. For example, write how there is a heartbeat that grows louder and louder until it echoes around the house. This builds suspense, grows the tension and sets a pace towards a scary event.
Dramatic Irony: Slowly reveal details to the reader, but not to the character until something scary happens. So a monster is hiding somewhere and the reader knows this, but the character doesn’t until its too late.
Cliffhanger: Build suspense and release it one big scene.