Get All the Feedback
If you have beta readers, a critique group and/or a content editor, make use of them! Get their comments on what worked for them and what didn’t. Then remember that while they collectively might be able to point out WHERE there’s a problem, their ideas for HOW to fix it may not align with yours. Then dive in!
Wait For It
The moments after your draft is completed may the the worst time to dive in and fix it. It’s often best to take some time, turn your mind to other stories, write something else, don’t write at all for a while. Leave that thing in a drawer until you’re less emotionally connected to it. When you no longer NEED it to be amazing, then you can start making it amazing.
Some folks prefer to actually rewrite the entire novel from start to finish. Now, you don’t have to go crazy – it’s perfectly fine to put the documents next to each other in two windows and do some cutting and pasting as well as reworking. But this method lets you focus on whether you really needed that scene, and on what genuinely needs rewriting as opposed to just tweaking.
As an editor, I have three modes of thinking that I use when reading an author’s work, and you can apply these to revising your own. I put on my content editor hat, and I’m looking at the overall plot structure, the character arcs, the logistics, the big picture. In my line editor hat, I’m looking at character voice, word choice, sentence structure, dialogue tags, and so forth. And finally, in my proofreader hat, I’m paying attention to spelling, punctuation, grammar, consistency. Trying to revise all three at once would be exhausting, so I make at least three passes!
Read Out Loud
Now that you’ve been through your novel multiple times, you’re familiar with where the trouble spots are. Read those parts out loud, if not the whole thing. Reading it out loud forces your brain into a different way of looking at it – you’re literally using different parts of your brain when you hear it than when you read it. This helps enormously to make your character voice work, your dialogue better and your sentence structure more straightforward.