That's easy right? A great book! Well, yes and no. Of course your book must be great, but most importantly, the first five pages must be amazing. If it doesn't pull an agent in right away, they'll probably pass. Worse yet, most likely its an intern reading your manuscript, looking for something specific the agent has told them to look for. If your manuscript doesn't match what the agent told them to look for you still have a shot at blowing them away and making them read it regardless.
So how do you snag them by the collar and yank them into the pages? First, don't pull your punches. Don't save your best stuff for later in the book. What good is it a hundred pages in if the agent or intern never gets past page four? When you're as popular as Stephen King you can save the good stuff for later (actually I wouldn't recommend it even then) but for now pour everything you've got into it from the begining. This doesn't mean throw in the kitchen sink. It means write smart.
At all costs avoid clique beginings. You know the ones, a telephone rings, it's a dark stormy night, or worse, an alarm clock goes off. Be original! Let them know right away that this is going to be something special. Try to introduce your protagonist right away, they are who the story is all about after all. Above all, there must be conflict and something to gain or lose. Conflict makes a reader want to keep reading. It can be small or it can be huge, it just has to be interesting!
For more great info on the subject check out editor Chuck Sambuchino's blog entry on a panel of agents who were asked what made them stop reading: http://ow.ly/163Ovs