Play to primal fears. The classic themes are isolation, being trapped, illness/injury, scarcity (starvation, low air supply etc.) and being hunted/eaten. Mix and match to taste.
The taking ordinary objects and making them threatening is also a classic manouvre, it's a way to connect the audience to the scenario, provide something they can empathise with - mirrors are used a lot, people can be tracked through their computers, the killer knows your home phone number etc. etc.
Take the latest season of Doctor Who as a good example - there was a crack in the wall, and if you got sucked into it, it cancelled out your entire existence. There were statues which began to move the instant you stopped looking at them, and they were hunting you. There were holes in the ground that dragged you beneath the earth where a race of lizardmen vivisected you while you watched.
Also, if there's a seemingly simple instruction/imperative that can be repeated ("Don't Blink"), that' can be pretty powerful.
Good horror devices are silence and repetition. If you have a good, subtly Wrong image or phrase, repeat that at key points in the story. Handled carefully you can build kind of a pavlovian response as people will associate the image with Something Bad about to happen. Silence plays into the Isolated and Hunted aspects.
