What does a prologue do?
Really though. What is its purpose? It will be the first thing a reader will read. Therefore, it should grab the readers attention. Hook the reader in with something juicy to make them want to read the book. A prologue is also good to describe a scene from the past that is important to your story. In Harry Potter, chapter one really should have been turned into a prologue. It was a flashback to when Harry was a baby. In a prologue you can also have a different point of view. If you're writing the book in first person, then a prologue giving away a secret can be very helpful.
Should I or Shouldn't I?
Don't use a prologue for a hook only. Here's what I've found. You make this great prologue that hooks the reader. They start chapter one and realize that it isn't like the prologue and are turned off. You're just creating double the work. Why not just hook them in chapter one. Then you only need one hook. Less work.
If you need to provide background information then yes do a prologue. A lot of times books jump right into the action. If there is a scene that is imperative to the story but can't be expressed except a flashback then yes, do it!
Flash forward. For example, Twilight. The "preface" aka prologue is a scene from later in the book. It is a pivotal moment in the book. It immediately makes you want to know how the character gets to that point. It hooks you yet also gives you insight.
What not to do....
Don't dump a bunch of random information there. This sets the pace for the whole novel. If you just dump all this back story then you're novel will seem boring. I would most likely not even go to chapter one.
Don't make it super long. It's a prologue one paragraph. One page. Max. It's setting the scene not actually a scene. Well, I guess it could be... STILL. Keep it short.
That's all I can think of for now. Here is an example of a good prologue, Twilight. Enjoy!
I'd never given much thought to how I would die--though I'd had reason enough in the last few months--but even if I had, I would not have imagined it like this.
I stared without breathing across the long room, into the dark eyes of the hunter, and he looked pleasantly back at me.
Surely it was a good way to die, in the place of someone else, someone I loved. Noble, even. That ought to count for something.
I knew that if I'd never gone to Forks, I wouldn't be facing death now. But, terrified as I was, I couldn't bring myself to regret the decision. When life offers you a dream so far beyond any of your expectations, it is unreasonable to grieve when it comes to an end.
The hunter smiled in a friendly way as he sauntered forward to kill me.
P.S. If you know any good books with prologues let me know! I have been scouring the web and it's hard to find some good ones.