Summarize the following:
A plot twist needs to be unique or, at the least, capable of surprising your readers. Remember that you are trying to get them to say, “I didn't see that coming.” Keep in mind the story you are telling and write something that is out of the ordinary for the narrative.  If a character is murdered in a story with five murders, the sense of surprise is gone, and predictability sets in. If a character has a special item that they have been using the entire story, do not have them use said item in your twist. The reader will expect that. Instead, see how your character does without their special item. Do not make your twist so outrageous that it is unbelievable. It must be a surprise, but a reasonable one. Although your story may have fantastical aspects, it still has to adhere to the rules that you have created. Keep in mind that the more “out there” a twist is, the more you will have to foreshadow and ground it in the reality of your fictional universe.  Be sure to foreshadow. A twist should not come out of nowhere. If your narrative takes place in a medieval world, do not have aliens show-up in your twist, unless you have given some clues that aliens are involved in the story. A noir detective story probably would not benefit from dragons, unless dragons are an integral part of the universe you are creating. In order for the scene to make sense to your reader, you need to provide adequate foreshadowing. Drop breadcrumbs throughout your story that hint at the twist to come. However, do not be too overt in your hints. Be sure to redirect suspicion, otherwise, the reader will see the twist coming.   If a character is going to reveal a secret identity, be sure to have hinted at this identity earlier in the narrative. When revealing a false perception, be sure to create your world in a way that provides brief glimpses of the true reality. If you premise the twist on a well-known gag or trope, it is likely that your reader will see it coming. Gimmicks can cheapen your story and make the reader feel tricked or deceived. You want your twist to heighten your reader's investment in the narrative, not undermine it.  Do not have the entire plot be the dream of one of your characters or the product of a simulation. Framing or nesting can add an interesting element to a story, but it has to significantly add to the punchline. It is unlikely that the reader will appreciate investing their time in a story that was an illusion. Avoid at all costs. The revelation that a character is actually the long lost relative of another character is very much a soap opera cliché. If you are drawn to this twist, think of ways to make it your own. Avoid having one of your characters realize that they are a ghost. This is another notorious twist that the reader will likely see coming and not appreciate. In fact, avoid all M. Night Shyamalan style twists.

Summary:
Sidestep the obvious. Keep it reasonable. Foreshadow your twist. Do not rely on gimmicks and clichés if possible.