A trend that is still making me cringe, despite numerous debates from other established agents and editors who have mentioned the downside, is the dreaded PROLOGUE.
Now let me first say, that as an author, I once had the despised prologue opening my first manuscript. I learned quickly that it just wasn't necessary. I will tell you the reasons so you can learn from mistakes, because as an intern, I am still seeing these ungodly beasts of teasing, non-important, rife with cliché pages.
So why do most agents, editors, and interns moan, gnash their teeth and bang their head upon the keyboard when they see the that single word atop the first page of a querying author's manuscript. It's simple; it means nine times out of ten - because sometimes there is a one percent need for a well crafted, necessary prologue - what is TOLD in those prologue pages is all backstory that can and should be carefully and craftily woven throughout the REAL story. Sometimes as an author working through your first several drafts you need that prologue so you can better understand your characters' motivations and goals, but I guarantee by the time you get the final editing and revising, its gots to go!
Now, as I mentioned OCCASIONALLY (and I mean sparingly) there arises a need for a prologue. What comes to my mind is a recent series of historical romances by Sabrina Jeffries. In the instance of her Hellions of Halstead Hall, the prologue was needed because it established a specific point in the past of the main characters and was essential to the over all mystery arc of the series. It was a specific point that each of the characters had experienced when they were children and it was better told as flashback prologue than by infusing the knowledge throughout the story. The brief vignettes of the past set up what each character knew about this specific instance and what would propel their character throughout the rest of their individual stories.
Now to the meat, the reasons you should avoid the prologue, or what I see when I read them. Many times a secondary character is used to tell something outside the scope of the character's journey. As an author, you may feel it is important to convey this episode on the page. DON'T. If the story doesn't start with the main character it can be a turn off. Many times these pages introduce the villain first and shows them in the throws of their devious, murderous, maniacal actions. As a reader, I expect the villain to be bad, the worst seed. I don't need a separate opening to TELL me this. An agent once told me, the villain who's evilness is just hidden under the surface is more compelling, more surprising. Think Jaws, she said. And it's true. Build up the suspense, don't tell us or show us right off the bat how evil the murderer or villain or antagonist is.
Another reason why your prologue is still clinging for dear life when you query. You're writing historical or high fantasy and feel that to sufficiently reveal your world, whether the court of Louis XIV or the hills of Mordor (yeah, just examples), you've written a dozen pages of world-building with characters that are non-essential or a point that starts way before the actual story does. I've found that many times this is the case. World-building is hard folks, but dumping it in a prologue is just lazy. Sorry.
Finally, there's the prologue that opens the paranormal (usually romance). In these instances the "different" character or the main character with "paranormal" differences is introduced in an episode that shows their differences outside the main story, or before, or flash-forward to the climax of defeat or mastery of said powers. It's best to just start your story where it should and reveal the powers as the character learn them. Or worse yet is the prologue that shows how "normal" these characters are before the proverbial poo hits the fan. Weave this in. Start where the story starts. PLEASE.
In the words of those who have lamented before me, it's probably best to just leave the prologue off your manuscript. It can always be added later.