"But now that I've written that dialogue, I know how to make it better."
"I've had a great brainstorm, but Marge has to be in her 20s and just starting her career, not the 40 year-old director she is in the first couple of chapters."
"I'm going to go back and change the setting to Guam, the whole thing will work better."  

Don't go back and revise.  Make a note in the margin or at the bottom of the page ("more details of the train ride... drunk teenager enters second stop") and keep going.  

Why can't you go back and fix something? Because you don't know how to fix it yet.  Yes, now that you've written the scene once you know better where it's going and how to squeeze more juice out of it (or get rid of more rind and seeds, to continue the metaphor...).  But actually you don't know because... you haven't finished the novel yet.  

As you write chapter after chapter, your idea of your own story changes.  You plan, and the way you wrote the first three chapters, has Freddy as the main character.  But by chapter four, you've already completed three scenes where Freddy's brother George is jumping off the page, and the story is really about him.  It's not until chapter six that you hit upon the complication that really ramps of the tension: Julie is pregnant.  

Why does this matter?

It means you're wasting your time with too early revisions.  There was a long conversation between Julie and George in chapter two.  They were talking about Freddy and the lawsuit against the company he and George co-own.  You can polish that conversation all you want, but it's a waste of time because:
- you didn't know that George was the real hero of your story.
- you didn't know Julie was pregnant.  
And so, by the time you're writing chapter six, that nicely polished conversation between George and Julie has to be blown up completely.  It's missing any sense that Julie is concerned with something that throws the lawsuit into a new light.  And your new conception of George's role likely means there are some breadcrumbs you'll want to scatter about his interests and not just Freddy's.  

Obviously there's a balancing act here.  If you introduce a new character, Millie, and you realize she's compelling in her own right - you may want to write her scenes for a couple of days.  Diversions are okay.  Some of them won't make it into your final draft, but some of them will open doors you didn't know existed and you'll find a great sub plot.  But this doesn't contradict our rule - you're not going back, you're going forward in a different direction.  

The key is, write towards you big picture, and keep going forward.  There will be plenty of time to work out details and continuity issues and polish in revisions.  

When you're writing a novel, keep writing.  Don't stop to revise, there will be plenty of time for that later.