Plot-resolving dream sequences
Whilst there is nothing wrong with a good nightmare or dream sequence that is simply present to show how a character is feeling, or to haunt them as dreams or nightmares often do, try to avoid using dream sequences as important plot points.
Having your detective realize who the killer is in a dream, having a huge plot point occur or having some kind of emotional resolution in a way that doesn’t happen in the book’s “real world”, can be a cheap cop-out if not done correctly. A great example of this is the huge non-battle at the end of Twilight: Breaking Dawn. It angered a lot of people and cheapened the emotional impact the battle would have had were it simply part of the plot with a proper resolution.
For those unaware, Deus-Ex Machina is a situation where your character gets themselves out of peril by a very well-timed convenience that simply would not happen in real life. Such moments in a book are often used to wrap up a story or situation where the writer is either too unskilled (or simply too lazy) to come up with a realistic idea for themselves.
So if your detective is stumped for ideas, and then an anonymous tip off simply leaves a note with the killer’s name on it shows up, that’s Deus-Ex Machina at work. It’s little, almost ridiculous miracles that move the plot along when the writer is out of ideas.
A great example is in Lord of the Rings. Whilst poor Frodo and Co. are forced to trudge across perilous lands for three over-long books, Gandalf summons a huge eagle to get himself out of a pickle. An eagle which, quite easily, could have simply flown Frodo to Mordor and dropped the ring off within a short amount of time.
The strong man, and weak woman /The strong woman, and weak man
In almost all action-based stories, we used to see the strong man being tailed by a quaking waif of a woman. He would be fearless, competent with firearms and have a repertoire of witty one-liners for use at any given moment. She would be beautiful, doe-eyed and completely useless.
The same goes for the more recent cliche of having a strong female character who can beat up men three times her size, and a bumbling fool of a man who isn’t good for anything.
If your plot and character building legitimately serves these tropes well, you might be able to get away with it. However, it is much smarter and well-rounded to give all of your characters depth, strengths and weaknesses that subtly play off one another and help them both to resolve the problem at hand.
The out-of-place culture shock
Unless your book is set in North Korea and your main character is a black guy somehow forced to live there, the idea that people of two different skin colours are completely shock-and-awed by one another’s proclivities is something that should be left with the black-cop-white-cop movies from the 1980s.
If used well, a culture shock moment can work very well (especially if, for example, your racist character who only surrounds themselves with “their people” realizes other cultures are not so bad after all by mingling with said culture).
If used cheaply, your book is going to be disingenuous from the start.
The time-convenient conquering of a fear, concern or doubt
One of the best examples I can think of right now is the black cop in Die Hard; he accidentally shot someone early in his career and for many years, he has been scared to use his gun whilst on duty, because he doesn’t want to repeat the pain and trauma. Naturally, at the end, he overcomes that fear in the blink of an eye to shoot the bad guy and save the day.
This trope is done to death, and it’s a good idea to avoid it. I’m not saying that your hero shouldn’t confront their fears or problems, but try to make it more subtle.
A good example of this working is a man finally calling his father after avoiding his calls for twenty years. This can be built up to over the book and the decision can be drawn naturally as the character grows.
A bad example is the girl who is horribly, deathly afraid of deep water diving face-first into the ocean without any issues or reservations to rescue someone or something in peril.
This is a tricky one. The problem with using amnesia sometimes is not that it is being used, but rather how it is used. You must be able to provide good reason as to why they don’t remember anything, and also carefully control what they remember; if your amnesia is too plot-convenient or ill explained it will fall into the Deus Ex-Machina category and leave readers feeling cheated.
In The Raw Shark Texts, Eric Sanderson wakes up with a case of total amnesia. The reason this works is because the very plot revolves around a “thought shark” called a Ludovician that literally eats a person’s self, which eradicates their memories whenever they wake up from an attack. It makes sense, and doesn’t leave you questioning why.
In my own novel, Don’t Eat the Waffles, there is a world-building point of amnesia in that nobody can remember how they died, only that they died; since the memory of one’s death may delay events, people don’t remember anything.
However, having your character wake up with amnesia for no reason and failing to explain just feels cheap. Also, selective amnesia (where the character has forgotten the last six months but remembers things when the plot calls for it) is the worst type of amnesia to use. If your amnesia is something that comes along with a head trauma, from drinking or drug use, be sure to research how it affects people, and how often (or fast) the memories will come back.
In the end, just remember that a cliche exists because it’s a genuinely common occurrence. However, if you use one in your book (and let’s face it, we’re all going to use one at some point), be sure to inject a little originality into it or play it out in a way that presents it in an original manner.