When I review manuscripts for aspiring writers, there is a problem that I see time and again in books that are right on the brink of being publishable. These writers have put some effort into the mechanics of their work, so there are very few errors of punctuation or grammar or spelling to point out. They've worked hard to develop their characters and plot, and they've written a complete manuscript.
(I find that actually completing the manuscript often translates into a writer who does everything else better--characters, setting, mechanics, and plot--probably because the discipline necessary to actually finish a huge project like a book carries over into all aspects of the work.)
The final hurdle that must be crossed to generate professional-level work is high, and it's hard for students to wrap their minds around, because it isn't as easy to point out as a misspelled word. Almost-publishable work generally lacks pacing. It isn't tight. And tight copy is like pornography...I know it when I see it.
So how is that helpful to you, who are so intimately entwined with your book that you can't see the forest for the trees? Well, first, you need to set it aside for a few days. Then you need to read it as if you just checked it out of the library on a whim. Read it for pleasure. If you're honest with yourself, you'll recognize that it feels flabby, somehow. You will be fighting this feeling throughout the editing process. Every time you read that book, you should be identifying useless adjectives and prepositional phrases that say nothing. Deleting such things should give you a feeling of great power!
On a larger scale, you must make sure that dry, boring exposition is not dragging your story down. Look at the opening chapter, in particular. How long does it take for something to happen? Beginning writers feel that they need to introduce each character and describe the setting and set up the action, all in the first chapter. Wrong! How many people would have watched Raiders of the Lost Ark, if the screenwriter had opened the film at the university where Dr. Henry Jones was teaching, then described the Ark of the Covenant, then put us on the airplane with him to get to the jungle where he wanted to find a golden idol? Oh, and the unrepentant scene-setter-upper who wrote this engrossing flick would have probably spent that plane ride describing the idol and the booby traps that awaited Dr. Jones when he found it.
Instead, we moviegoers were plunged into danger and mystery and tarantulas and a honkin' big rolling rock before ever really meeting Indy. It felt like we were dropped out of an airplane. And we loved it.
When do you introduce your characters to your readers? After they're hooked. That, ladies and gentlemen, is what flashbacks are for.
I was still learning this when I wrote Artifacts and Wounded Earth. Their opening chapters morphed into something complete different as I edited. Most notably, while editing both books, I reversed the first and second chapters, then I slashed the bejesus out of the original opening chapter. As you develop in your craft, you will find that you almost always need less exposition than you think you do.
Please, please, please do not describe your alien villain's elaborate military uniform to me before he whips out a blaster and threatens somebody with it.