One of the things about doing Book Doctoring, as I'm doing at the Getting Published conference in October, is that you get to see a lot of beginnings of novels. I'm starting to think that a great many aspiring writers believe that a book isn't properly dressed without a prologue. And, to be frank, most of the prologues I see aren't earning their keep.
It's not that they're never the right thing (I have one in A Secret Alchemy, and a sort-of one in The Mathematics of Love), only that usually whatever they're supposed to be doing would be better done by other means. And because of that, agents and editors regard them with a jaundiced eye, shall we say: their experience tells them that it's not a promising start. In the interests of making a better book and improving its chances out there, I'd suggest that you ask yourself very stringently, "What is my prologue trying to do?" and then "Could I do that as well or better by other means?"
1) Is it really Chapter One? A prologue is something which isn't part of the main plot/structure/chronology of the novel: it happens in a distant time or place from the main story, it's a different voice or a different character from the main thing. If it's none of these, then it's Chapter One and should be called as much.
2) Is it trying to start the novel with a bang? If the main story starts too quietly or with stodges of back-story which you're convinced the reader needs to know, then bolting a bit of drama onto the beginning isn't going to cure it. Instead the reader will wade resentfully through the stodge waiting for the action to start again. What will cure it is getting enough bangs into Chapter One... or recognising that the real start to the story is curretnly called Chapter Four.
3) Is it trying to create atmosphere or contribute to a theme? Again, if it's an atmosphere that matters in the novel, then it needs to be there for the "real" start: Chapter One. If it's theme, then I see why you're doing it, but I'd suggest that the need for the "real" story to start as quickly as possible is more important. Theme and atmosphere on their own don't contribute to narrative tension.
4) Is it trying to set up mystery and suspense: narrative tension? At the beginning of a book we have no investment in the characters and their fate, so simply being mysterious about stuff - hinting, implying things - isn't enough (it may seem enough to you, because you know what the hints and teases are hinting at. We don't). Narrative tension doesn't, fundamentally, come from information being hinted at but witheld, unless we're also given a real desire to keep the incomplete information on our mental clip-board, which breeds a willingness to keep reading, without the whole picture, until the information is explained. Narrative tension does comes from instability, as I was explaining here, from us caring about what happens to characters-in-action. If you do want to hint things, then what you do say has to be full of delicious, fascinating stuff: not witholding something so we only get a glimpse but, as Susannah Rickards puts it, like dropping a trail of sweets to lure us on into the forest.
5) It it trying to explain backstory? If it's just straight stuff about their past, I'd put money on the reader not needing to know it: almost every novel I see in manuscript (including my own...) needs less explaining of backstory than the writer thinks it does. And if we do need to know it, it's almost always better to weave/drip/slide it in to the main narrative, than to give us a wodge at the beginning, and hope that we'll hold onto it. Again, it delays the caring-about-the-character. I think there MAY be a case, if the prologue is, say, a scene of a mother or father leaving home for ever, kissing the sleeping babes, and the main novel is about one of those babies, grown-up, trying to track the mother or father down. But a prologue like that has its own instability: as with putting a murdered body on page one, we KNOW that something like a missing parent is unstable: something has to happen. That's proper narrative tension. But if you're putting in the prologue purely to explain your series detective's dysfunctional character, not as an element of character-in-action (the plot doesn't include tracking down her parents, say) then that's the kind of backstory which is much better dripped in, if it's put in at all.
Fundamentally, the task of the opening of a book is to get us involved in the characters-in-action as quickly as possible, so we keep reading: I explored some of the ways you can do that here. With a prologue, our getting to care about both the characters and the action is delayed. We invest in the world of the prologue, but then the pause-button is pressed on that, while we're expected to start all over again with a new world. And yet we can't even (crossly?) let go of that first world, because our readerly savvy knows that at some point it's going to be relevant again. That's a cost in readerly irritation, and/or un-involvement, which must be outweighed by the benefit that the prologue can offer in return: are you sure yours does?
On a final note, if you've tackled all these questions and decided that the prologue really, really is the answer, consider nonetheless taking it out if you're about to submit the first few chapters to an agent or a publisher. If the sample you're submitting doesn't get the reader to the point where the significance and purpose of the prologue is clear, then leave it out of the submission. If you're asked to submit the full MS, then put it back in.
PS Since I posted this, I've had a big lightbulb moment, courtesy of Pixar's Andrew Stanton, which I blogged about here. He talks about how the title and opening of a story makes a promise to the reader, that sticking with the storyteller will be worth the journey, and, essentially, the story needs to deliver on that promise. So in thinking about your prologue - and more widely about the opening of a story, which I blogged about here - you do need to bear this in mind. The the first thing the reader reads - whether it's labelled Chapter One, or labelled Prologue - is essentially a statement of that promise, so even if it is a separate, prologue-y sort of chunk, it needs to be stuffed full of promises about what the main story will be trying to deliver.