Have your book edited 1 or 2 times—There’s not much worse than putting out a manuscript rife with errors. There are plenty of editors out there to read your manuscript, find typos and grammatical errors, and make suggestions on plot, style, tone, etc. You’ll be surprised at how much they can improve your work. But even the best of editors isn’t going to catch everything, and even the best make mistakes or must make judgment calls that aren’t necessarily the best decision for the manuscript in question. I generally have my books edited at least twice if not three times. (This is in addition to having the book read by actual readers as a test audience.) This gives the opportunity to catch mistakes. If your manuscript makes it through three rounds of editing by professionals, there may still be mistakes but there will be far fewer. And I’ve found that three rounds of editing gives the editors and opportunity to come to consensus on changes over which there may be disagreements. One editor may say use a comma. One may say use a period and make a new sentence. One may say use a semicolon. I generally put them together and ask them to come to a consensus of what would be most acceptable in the publishing world as opposed to stylistic decisions.
But the editing process isn’t just about finding typos and grammatical errors. They’re going to identify inconsistencies and gaps that need to be corrected before the manuscript is published. They’ll identify plot and character developmental issues and perhaps make recommendations on how to improve for a better finished product. Most importantly, a good editor is going to give you impartial, honest feedback about the manuscript—constructive criticism that only makes an indie author’s work better and that helps the indie author in the future.