You need to give your thought process a break between first and second draft. Ideally this is a couple of days, but even 15 minutes of playing Angry Birds or talking to your spouse about where to put the new climbing roses breaks your thinking process enough that when you go back to it, you’re much better able to see whether your narrative arc holds together, and what you don’t really need. Read it aloud to yourself before you start rewriting: What sounds wrong?
That’s Megan McArdle, senior editor for The Atlantic, talking about the best self-editing advice she knows. For years, I operated on a “Write it and get it out there!” plan, just churning out material; I’ve started re-writing and re-working material recently, and the “you need to give your thought process a break between first and second draft” thing is so amazingly spot-on for me; even just something as basic as having dinner can allow me to break through a problem that seemed insurmountable before.