There are two things you should do:

1. Wait long enough for your mind to get away from the ‘writer’s mindset and into the ‘reader’mindset. That way, when you read, it’ll be as if a reader was reading — and everyone knows that readers find all kinds of mistakes, typos, etc. If you are in ‘reader’ mode, so will you.
2. Don’t just edit your work once. Every time you sit down to write, go over what you’ve already written— or the latest chapter/few pages/part if you are writing a long, long piece— to look for flaws or ways to say it better. Every time. No exceptions. I guarantee that each time you do this you’ll find something that you missed earlier. Do it. It’ll work.