How to create and film ghost special effects
There are so many interesting ways to do this.

One of the easiest ways (in-camera, no post production work) is to use a piece of glass. Film Riot did a tutorial on it, they explain it best, see below:

                
            

Or you could go with a bit of post production work, after effects being my go to tool that I use. Add a background video layer (without ghost), then add your ghostly image, same shot, but this time your ghostly image is brightly lit and everything else is dark. Then change the mode of the ghostly layer to 'add' (or other to create different effects) to create the ghostly effect. See tutorial below: 

                
            

Hope that helps your ghostly scenes.