This is one of the biggest mistakes of many self-published authors and why it's number one on my checklist. You must hire an editor. And not your friend or family member, but a professional.
The problem with editing your book yourself is that it's yours. It's your baby. You're too close. You can't effectively look at it from the outside in and pick out any plot holes or character issues. You can't easily say, 'this would be too confusing to a reader and should be explained.' You know what you meant, and although it may be quite obvious to you, it won't be to them. Perhaps you explained something in too great a detail and won't realize you'll do nothing but bore your readers for 10 pages. You've read the words so many times you see what you want to see, not what is actually there. Fresh eyes will pick out wrong and misspelled words much more efficiently. An editor will be able to give you advice on punctuation and sentence structure. They'll help smooth out some awkward dialogue and point out any inconsistencies.
Why working with an editor is so important is that a lot of the errors and problems an editor will find, your readers will find as well. This is one of the main reasons there is such a horrible stigma on self-published works, and rightly so. Your readers are paying money to immerse themselves in the story and world you've created. Respect that and give them a polished and professionally edited book. No matter how amazing your story is, it won't be if it's executed poorly. If it's filled with errors, it's difficult to read and takes away from the experience.
But it's not just about respecting your potential readers. It's about respecting yourself. You worked long and hard on your novel. You deserve to have the most professional work published. Part of being an author, whether traditional or self-published, is building an image for yourself. Let your story and writing speak for you, not a plethora of mistakes. You put forth the effort. You deserve the best to show for it.
Now, after this long spiel, I want to make a couple of things clear. I'm not saying to never edit your work, nor am I saying that even with an editor your book will be perfect. (Let's face it...is any book ever perfect?) Every time you go through it you will find things to change, but you won't catch everything. Even in major publishing houses, mistakes slip through. We're only human, after all. But our goal as writers, especially self-published, is to put forth the best, most cleanly published book we can. Not only will you help your sales, but you will also help the sales of other SPA's. Every well-edited book will help release the stigma that all self-published books are crap. Show the world that your book deserves to be read just as much as Brandon Sanderson's or George R. R. Martin's.
What are your thoughts? Have you self-published or are you thinking about self-publishing? How did you handle the editing?