Another technique that it very effective is to lull the player into a false sense of security by having them repeat an action several times, then jumpscaring them on the last time.
For example, in Outlast, I very distinctly remember a scene where you have to walk past a guy who is sleeping in a wheelchair. When I first played it, and in several let’s play videos I have seen, the first time walking past him, you are very uncomfortable. Everybody expects him to jump up out of the wheelchair and attack you. He doesn’t. 5 minutes later, when you have to walk by him a second time to get back to where you were, no one expects him to do anything. So they will just blindly walk right by him. When he jumps out of the chair and attacks you, it is a lot more terrifying because you were not expecting it.
Overall, anything you can do to make the jumpscare unexpected makes it scarier.