Darkness- Investigating at night or in a dark room is fun; the creepy factor puts one on edge and in the mode to catch something.  You may consider it a way to enhance the other senses besides sight.  Also, it is the way they do it on TV.  It is in this type of condition that many report seeing things they claim are paranormal.  In most cases, the personal experience that people bring out of an investigation can be attributed to your eyes playing tricks.
Your eyes have two types of sensory receptors, Rods and Cones (1).  The Cones detect the brighter light and color and are centered in the retina inside the eye.  The Rods pick up dim light and less detail and they are more towards the periphery of the retina.  So, you tend to see very dim light out of the corner of your eye but you can’t focus on it and the detail is lost.  So many ‘sightings’ are from the corner of your eye and this is a reasonable explanation for these occurrences.  The lack of detail and the inability to focus should render these sightings as very questionable possible evidence.
Your eye is also susceptible to after images (2).  You stare long enough at one spot and them move your attention to a different spot and you have “ghost” images lingering that resemble the last spot where you were looking.  In low light the brighter images will burn in and look like shadows when you move your eyes to a new view.  Many mistake this for shadow people, apparitions, anomalous lights and movement in the dark.  The only thing that sitting in the dark increases is your chance of false positives which are claimed to be paranormal.
Here’s a fun sight that teaches about eye tricks. http://faculty.washington.edu/chudler/chvision.html

Technical Jargon- For many in the field, especially those with little or no scientific training, a big set of words may be good enough proof of the paranormal.  Technical terms and descriptions of natural processes are often mistaken as expert paranormal knowledge.  There are no experts in this field.  Sounding smart doesn’t make a good investigator, common sense does and common sense should tell you that big words do not equate to success in finding ghosts.  It does lead to more false positives as they convince people that naturally occurring events are paranormal by sounding official.  Many self proclaimed celebrities offer the same value, they are revered for their expertise yet there really is no formal training or knowledge that makes someone stand out so much that they are beyond reproach.

Photography- There are so many articles out there on this topic.  It is also amazing to see and hear how many people know little or nothing about how a camera operates and they don’t care to learn.  They say you just point and shoot and that’s all they need to know.  Very wrong.  A digital camera is possibly the best tool for success but remember, in this article success means false positive results.  We have seen hundreds of pictures, maybe thousands.  Many with an anomaly, but the result was explainable in 99% of them.  I don’t even want to get into the truly manipulated photos with software or a phone app (3).  It’s too frustrating that these are accepted by many as proof.  Artifacts from flash photography such as dust orbs (4) (100% of orbs caught on camera are dust or similar particles or moisture or insects), long exposure times in flash mode or non-flash mode, night time settings and reflections of objects in the subject field are all mistaken as proof.  Still, those taking these photos do not agree of course.  The other 1% or so is likely a photo aberration; manufacturers know that a small percentage of artifacts from the camera electronics or mechanics happen in digital photography especially in low light. Auto-exposure just gets it wrong sometimes (5).  So, although many feel their most compelling piece of evidence is a photograph, we throw out all photos.  No orbs, no light streaks, no reflections in mirrors or glass and no apparitions.  Not one makes a convincing argument for paranormal.  There are too many possible problems and it’s not a reliable source for data.  In the sciences if it’s not significant or there’s a large chance for error it is tossed.  Toss your photos, especially the orb photos!  Here is an article explaining how to prove orbs are dust, etc. http://nyackparanormal.com/BoP/burdenofprooforbs.htm and another explaining the streaks caught in photos http://www.nyackparanormal.com/LibraryArticlesStreakedLight.htm.  Just more false positives.

EMF and Other Meters- Electromagnetic Field meters measure EMF.  That’s it.  Oh, they sometimes pick up stray radio frequencies too.  So is this a good tool for ghost hunting?  Yes, false positives again!  Many buildings have wiring, stray EMF from high tension wires outside, emergency response walkie talkies from outside, cell phones or maybe investigator walkies inside and other investigators' equipment.  There are many sources for spikes in the EMF including natural ones(6).
Who ever claimed it can be used for communicating with the dead?  It was stated on TV, the K2 Meter on Ghost Hunters.  Soon after it aired they were for sale and they sold well.  K2 meters are very susceptible to RF and few or no groups use an RF detector alongside it, why?  Because it will cut into their false positives.  They don’t want to know if it's RF setting off their meter, they want it to be a ghost.  Here’s an article about the K2 http://www.nyackparanormal.com/k2blog.htm.
There are other pieces of equipment used for measuring different parameters such as temperature, humidity, pressure and radiation to name a few.  A radiation detector will also give false positives.   Background radiation is everywhere, from naturally decaying elements in the ground to cosmic rays coming from space, its normal (7).  But turn on a meter and show some counts because we are looking for false positives.  False positives are the result of lack of knowledge or a misinterpretation of the findings.

EVP- We have found many audio recordings very compelling.  One recording we have from a burial ground is astonishing and we have difficulty explaining it away although we still don’t label it a ghost.  EVP seems to be the most widely published form of possible evidence.  Most groups come away from every investigation with at least one, most with multiple.  We try to listen to as many posted EVP as possible and most of the time it is really an unintelligible sound that is likely just room noise.  Many groups do not tag or verbally label sounds that are made by outside sources or unexpected noises in a room.  Stomach noises, feet shuffling, handling of the recorder and even participants' whispers are mistaken for spirit voices.  These are then manipulated and enhanced until it resembles something intelligible.  TV shows do this too for ratings.  Also, most post what they think the ‘words’ are which then influences the listener.  EVP analysis is full of false positives which lead to more claimed successes.

Ghost box- The ghost box in many forms has been around for years.  Generally all versions do basically the same thing; they scan radio frequencies so you pick up very brief snippets from each broadcast.  Most use AM, it provides more white noise which some believe is the audio fuel needed for spirits to form words through the radio.  It is also the band that provides much more talk radio which supplies words that many interpret as spirit communication.  An article here http://www.nyackparanormal.com/GBox.htm provides some detail about the effect this has on those utilizing this tool during investigations.
Many claim the snippets are too short to be heard as full words from the broadcast.  It is written in reference that human English speech is spoken at a rate of approximately 110-150 words per minute.  Audio books shoot for 160 words per minute (8) and radio broadcasts are likely a bit higher.  Even at 150 words per minute that’s one word every 0.4 seconds or over two words per second.  By a high level of probability you will string together words, partial words or sounds that will be interpreted as a sentence or phrase. This is known as Audio pareidolia (9). If you ask the box questions first, you are already predisposed to hearing an appropriate answer. Bingo, more false positives.

Flashlight Communication- This is a well known parlor trick.  A Maglite, almost always a Maglite (other types may work the same), you set it just at the cusp of off and on and it will cycle on and off because of the temperature differences as the head heats up from the lit bulb and cools down when the bulb goes off (10).  I had two cycling on my desk for over a half hour before I was too bored to keep watching them.  If you are good at it you can figure out how long it takes to cycle and ask a question just before it changes, or you can ask, "how many?" and count until it changes.  You could also just keep asking the question until it changes, all these techniques are used.  I want to throw a chair at my TV when I see it used by Ghost Hunters.  Try setting it and leaving it, don’t ask questions. Nothing paranormal about this at all but it guarantees success once again.

Psychics- Psychics are an excellent source for success.  They always come up with something.  I am not judging any single psychic’s ability here, but they do guarantee results. If the information provided is unknown, then there is no way to substantiate it and believers accepted it, as is.  If the information provided is spot on to the history of the place then there could have been prior knowledge.

Ovilus and other word generators- Here is an item that even the creator claims is for entertainment purposes only (11).  It is a box that generates words.  A spirit is supposed to learn, on the first try, how to manipulate the environmental factors so they can choose the right word in the electronic memory of the box.  Sounds easy, especially for the technically advanced spirits of the 18^th century.  This is great though, another item that guarantees success! On TV they edit. There are likely ten misses for every word that sounds like it may apply to the situation; they won’t show you the misses.  Also, those listening and watching are predisposed to make sense of the words that come out of the box.  For instance maybe the word ladder comes up, someone may interpret that as the means the spirit needs to climb out of the grave, a stretch but it now fits.  Success once again.  Any word generator will eventually sound right by coincidence, its not a telephone to the grave.