I got asked on Twitter about my stance on prologues. I thought I'd expand on what I said there over here.
I am not intrinsically against prologues. HOWEVER, the vast majority of prologues I see aren't necessary. A prologue should entice the reader, make them curious, or hook them emotionally in some way. It might introduce events that are significant to the story, but somehow separate-- happening before the main story, or to different characters that the main protagonists. If it is happening immediately before the story starts, to the same characters, from the same perspective, then I question why it isn't just chapter one.
Two examples of effective prologues spring to mind-- George R. R. Martin's A Game of Thrones is one. It opens far to the North, with the Night's Watch confronting a creepy Other. Then the main narration sweeps us south to Winterfell, and the Starks. The significance of the prologue is not immediately evident, but when you read about the decline of the Night's Watch, and that Winter is Coming, the prologue tells you that that's some serious bad news. Also, and I'll come back to this, it's 11 pages long.
A prologue separated by time is in C. S. Friedman's (awesome) Black Sun Rising. The prologue opens with a woman coming home, to her husband, in whom she's noticed a change in personality. It's worse than she thought; he's killed their children, and then he kills her. Then there's chapter one; a totally different character, and , we find out, centuries later. As the book progresses, we run across Gerald Tarrant, the guy from the prologue, who sacrificed his family for immortality. We already know that he's willing to do just about anything for power, and that he's not a nice guy. The prologue establishes that more powerfully than talking about backstory would, although that's a choice that Friedman could have also made, and I'm sure would have done well. This prologue is 8 pages long.
So those are two example of prologues done well. They are efficient, direct scenes that establish material that's important to the story, but not necessarily part of the main story. But, and this is important, these scenes contain everything that a broader scene must have-- characters who want something, who have a stake in the action, and conflict. They entice. They intrigue. A prologue gone wrong does none of those things.
Prologues I see that do not entice: the author knows that a big infodump in the middle of the story bogs it down, so rather than spicing their narrative with worldbuilding, they splat down a big lump of it at the beginning. This doesn't work. Readers connect to characters, not worldbuilding. They won't care about the history of the world until it's significant to the characters. A prologue with all the details of the history, mythology, and geography of the world, but no specific people is most likely going to be boring. I will probably skip past it to see what your first chapter looks like, but a reader picking it up at B&N probably won't, and the person who gets to the end of the Kindle sample will shrug and delete it. And I'm looking at your first chapter anyway, but my confidence has been shaken.
A type of prologue that can work, but that makes me wary: the jump-ahead scene. You have a scene from later in your book. It's totally awesome! It's the pivotal moment of your book! Your protagonist is hanging by the fingernails from a cornice! It's LIFE! OR! DEATH! So you start with that scene as a prologue, then rewind back in time and continue forward until you catch up with that scene. There are a few risks with that type of scene-- one, if I don't know the stakes for the character, why do I care that he's hanging by the fingernails? Two, the impact of the scene might be lessened by the time the reader gets there. Three, it makes me wonder if you're not confident about your beginning. How it can work: if the stakes are significant and strong enough to stand alone, and if by the time I get back to the scene, I have enough additional information that it changes the meaning of the scene. (Not a prologue, but a great example of the meaning of a scene changing: Sarah Waters' Fingersmith. She revisits the same scenes from different perspectives. Because of what the different narrators know, the meaning of the action is completely different the second time around, and it works brilliantly.)
A final thought on prologues: keep them short! Again, you want to entice your reader, tease them with hints, not bang them on the head with a lump of exposition that doesn't fit with the story.
Other example of prologues that work or didn't work for you are welcome, and of course, feel free to tell me I'm wrong about long prologues or anything else.