Ghosts are basically fields of residual bio-electric energy.   All electricity follows a circuit!   Psychic energy, though it operates slightly differently than the electricity that runs your toaster or television still follows a patterned circuit.

Just as when lightening strikes a tree, there is no physical circuit that can be observed with your eyes, but science knows that the strike will generate from the most intense source of energy build up and then reach out for the nearest object.   Once this energy has found the closest object it will strike and follow it, until it's energy is expended or grounded.   The energy required for a haunting also follows a circuit, but whether this results in a one time manifestation or a repeating or cyclical event will result in it's energy source.
When we are frightened, our bodies have a magnificent response that science has termed "Fight or Flight".   We know that during the fight or flight response we become supercharged to either "fight off" the threatening offender or to "run" like hell.   Either choice will be in our best interest so that we may escape or protect our lives.   Spirits can then use this high power fuel that we ourselves are providing, for thier own purposes.   Just as they draw energy f rom the atmosphere which results in the famous "cold spots", they can draw energy from this high octane fuel that fear provides.   It is easy to understand from this, that if you always respond with the fight or flight panic, you will be setting up the scenario for repeated manifestatons.
Knowledge is the key to desensitizing fear.  

One Time Events
Are just that, an event that happens once and then it is over!   There are many reasons some we do not understand that may trigger a one time event.   A frequent occurance is to see a loved one at the moment of thier death.   Other reasons may be an anniversary, an emotional family event such as a wedding or a funeral, perhaps the birth of a child.   You must also take into consideration - electro-magnetic fields, or maybe a violent storm or a lunar phase.   Although we may never, and most likely will never know what caused the event, it is sufficient to just accept it and move on.   In most other cultures these events are a part of everyday life, and one would be considered "blessed" to have experienced it.   In our culture it has become a dreaded thing, associated with fear and panic.   I wonder how much of our view was inspired by Hollywood?
 
Repeating Manifestations
With repeating manifestations, most usually start out as knocking, rapping or h earing someone walking. A little noise here and there, or maybe something appears lost but basically pretty doggone uneventful.   At this stage, most folks do not push the panic button nor do they believe they are being haunted, instead they rationalize the sounds away.   Many times after a few episodes it goes dormant.
But in a situation where stress and tension can be common place, the manifestations may escalate particularly if there are teens involved.   Please keep in mind, that Poltergeist activity is NOT a haunting - but is associated with the turmoil associated with adolescence, and is a whole different topic for another discussion.
Emotional energy can also cause an entity to temporarily deviate from its circuit and pattern of haunting, appearing in rooms and areas where previously it had not been observed. Emotional energy is theorized to be an entity's primary power source although the manipulation of and integration with electro-magnetic fields can also be assumed to be a source of their manifestation abilities.