It's probably poor form to write a post that's basically "what she said" and point at Sarah's latest post, so that's where I'm starting. So, "what she said".
Now to get into the meat of things - every story has its own rhythm, but there are quite a few things most of them have in common - unless you're writing "literature" of the suckitudinous "crap happens, but you don't care anyway because there's nothing admirable and nothing really matters" Pulitzer Prize-winning flavor (Seriously? Has anyone ever read a Pulitzer-winning book? The titles are enough to make me gag).
Think of your plot kind of like a backwards rollercoaster, where the lows are the quiet points, and the highs are where stuff really gets going. The beginning introduces the characters and problems, kicks the characters out of their normal life, and gets things rolling. This is the only place where you're allowed to use Deus Ex Author, although it's better if you can do it by Act of Antagonist or Act of Dumbass Character. If you do use Deus Ex Author, try to make it something that's a more or less expected or predictable thing in the universe of your story.
Everything after that should be caused by one of your characters, however indirectly, and there should be a series of heights and lulls. The slower sections are where character development and even stopping to admire the scenery can happen (although it helps if the character admiring the scenery is doing so for a purpose, like scanning for enemies or admiring the lady who's providing human scenery). They're also where you drop in the foreshadowing and the threads that push towards the next height.
What I've found is that there's usually a semi-climax partway through - anywhere between 1/2 way and 2/3 of the way through. Up until then, the heights get higher and more intense, and the lulls get shorter and offer the main character less respite. Typically, the semi-climax should be the most intense point apart from the climax, and the drop after it should be pretty steep and leave the main character in a state where there seem to be very few choices. This isn't what the Hero's Journey terms the Black Moment, but more of a pre-taste of it. Things are bad, there doesn't seem to be any hope, but the main character pushes on for whatever the reason. He/she should lose something that matters here.
After the semi-climax and the Swamp of Despair, there's a longish lull - not as long as the start, but longer than there's been for a while, then the cycle of ever-increasing heights and shortening lulls resumes, usually with steeper downslopes after the heights, and dips back towards the Swamp of Despair. Meanwhile, the climax looms ever-larger - it needs to start looking steep and ugly during this section.
Somewhere around the last 1/4 to 1/5 of the book or thereabouts, you move into the part where all hell breaks loose. This is the Black Moment where everything seems lost and there's no way out. The undead are everywhere, the cavalry's not coming, and you're alone. You get the idea. Here your character decides that he/she can't back out now, regardless of the cost. In romances, it's when it seems impossible that the couple can ever be together.
Then the climax should hit, hard. You're into the final battle and there's no time to breathe. This is the highest peak, the big climb, and right at the top is when finally it goes right. The rest is rather the like the afterglow, where you tie up the loose ends, clean up the mess, and leave everyone satisfied and - hopefully - happy but wanting just a little bit more. For sequels, rinse and repeat, but with higher stakes.
A few good examples: the first three Anita Blake books. The pacing in these is pretty much dead-on, if a tad predictable (Yes, I looked at how much book was left and figured all hell would be breaking loose within the next few chapters). Dave Freer's A Mankind Witch - note how the crises get bigger, and how Dave handles the quiet times. The first three Harry Potter books.
I should add that I haven't really studied this kind of thing: for me it's more of an instinctive thing. I can feel when I need to up the pace, and when I need to slow down and take a breath. I suspect it comes from reading damn near anything I could get hold of and absorbing plot structure more or less the way I absorbed spelling (one of my nicknames is 'walking dictionary' - but I sometimes need to see the word to know if it's spelled right). It's a bit like riding a bicycle or learning to drive - after a while you build a feel for it and your subconscious can short-circuit the conscious reasoning and just do the thing (Yes, I also have a lot of practice writing crap. I suspect most of my million words of crap were written before I ever got hold of a typewriter, back when I was writing longhand in notebooks. And going through pens like they were going out of style - and yeah, it is crap. I was cringing less than six months after writing a lot of it).
So does this way of looking at it work for you? Who else do you think handles middles and pacing well?