Proofreading. In the words of Ringo Starr, it don't come easy.
It's amazing how difficult it is to catch every error in a document. Your eye just skips over them because it knows what should be there and just assumes it is. Proofreading any document is difficult enough. Proofreading your own work is even harder.
That's why you should always find someone else to read your work. If you can't afford a professional editor then get a family member, friend, or colleague to read it. You'll be amazed at how many errors they are able to pick out. That's because it's much easier for someone who isn't familiar with the text to spot errors.
Furthermore, listening to someone else read your work allows you to hear where they get tripped up. If your reader struggles with a sentence, that's a strong sign your sentence could use some work. If not, you're in a good place.
If you can't find someone to read your work, use technology. For instance, our writing improvement software catches typos, inconsistencies, and even hard-to-read sentences.