Ghostly Images
To make your subject appear transparent and ghostly, set your camera exposure to a longer amount of time, like 8 seconds. When you take the shot, count out loud and have your subject duck out of the picture early, like at 5 seconds if your set to 8. Shooting the pictures at dusk will help keep the images from being blown out because you need a darker setting to avoid overexposure.

Double Exposures
Using the same technique of a long camera exposure, have your subject move to a different spot half way through the exposure instead of leaving the frame completely. Then their image will be repeated in the same picture.

Create an Eerrie Scene
For a scarier effect, shoot in black and white mode or desaturate the color image in a photo editing tool afterwards. Black and white photos create a feeling of timelessness and mystery. Try dressing your subjects in vintage clothing like top hats and old white dresses. Add scary props like an ax to complete the frightening scene.