Once you've had time to take a much-needed vacation from your work, it's time to return to it. Use these steps as guidelines for working on your book during the editing phase. Remember, you can only edit so much, and when it's over, you'll feel it. Good Luck!
This seems silly to say, but you need to take your book off the shelf and read it that same way that you would read any other book. Do not edit the book. Instead, read the entire book start to finish.
When you are through, read it again, except this time, write all over it, looking for big picture errors. If you don't want to write on it, then write notes inside a notebook. These aren't small-scale edits. Don't fret over spelling or grammar. Focus on characters, setting, plot.
When you have done this, set the book back up on the shelf for a week. Seriously. You are in it for the very very very long haul.
This is my favorite part of editing. On the second or third pass at your book, work through it and cut out all of the extraneous material. Start with the little things: extra words, fluff words...any word or phrase that doesn't lend to your story.
When you are done cutting words and phrases, you may find that other parts of the story are worthy of cutting. It's okay to cut an entire chapter or conversation. Once you do so though, make sure the connective tissue is still there on both sides of the cut. Don't make a jump that your reader won't understand.
Once you've worked through your manuscript, cutting and adding in whatever needs to be so, read it again and identify pacing issues. If your book is all slow, consider what you can do to very it. If you push through too fast, you may want to work in some slower exposition.
Of course, if you are writing a fast-paced book, then make sure there are no lag points. If you identify a lag point, then you've identified a place in your book that needs revision. Do this throughout your book -- flag revision worthy spots.
Once you've done that, you will be ready to revise. Do that :)
Make sure every character has an appropriate ending, even if that is an unresolved ending. Make a list of every single character and trace their story arc. Make sure that you haven't forgotten anything.
Check to see that minor characters are minor...sometimes writers can work too hard to make a minor character stand out, and that will leave your reader wondering if it was a major character...work through those issues.
Once you have edited your book two or three times, I recommend sending it out to several independent reviewers. You can hire them, or if you have a few readerly/writerly friends, they can help you. A reviewer should be able to read your work with an unbiased eye and deliver a solid report on the strengths and weaknesses of the book.