These are my opinions; not necessarily anyone else's.
1. Don't obsess on the opening.
Some people spend all kinds of time working on the opening to their book when they're starting the first draft. Usually, that's a waste of time, because by the time you finish the book, you will likely have a totally different opening in mind. So put something down and keep writing. Go back to the opening when you have the first draft done.
2. See that the rest of the book delivers what you promise.
The opening has to be connected to the rest of the story. In other words, you can't just give a rip-roaring opening and grab the reader and then have the book turn into something entirely different. If the book is a romance, it has to have a opening that promises a hint of romance. If it's a mystery, there should be a glimpse of trouble to come. And so forth.
3. Sustain the mood and the feel of the opening for at least the first couple of pages.
Too often, the inexperienced author has a great first couple of lines, but then switches into an explanatory or descriptive voice, as if all you need to do is hook the reader and then you can hit him over the head with all the background details you think he needs to know in order to understand the characters and the story. Instead, keep the reader guessing a little bit longer, and bring in those details slowly and carefully. In other words, you may have the reader hooked with your first couple of lines, but you have to reel her in slowly and carefully.
4. If you decide to have a prologue at the beginning, make it a real prologue.
In other words, I really am not a fan of prologues that are really part of a scene later on in the book, and are put at the beginning only because they bring some immediate action. I feel it's a sneaky way to try to grab the reader, and you're better to have less "action" and more of a genuine opening.
The two exceptions are:
A. When there is actually something that happened some time before the story begins that has a direct impact on the story, and is important for the reader to know before the story (and can't be readily worked in later).
B. When the story is being told by someone in it at a later date – as for example, the adult Scout narrates the story in To Kill a Mockingbird – and the reader needs to know this. And that's it for today.