Know the basics, or know where you can reference them: dictionary, thesaurus, The Elements of Style (Strunk & White), How to Write Your Best Story (Martin), Formatting & Submitting Your Manuscript (Sambuchino), etc.
Be honest with yourself. Put some distance between yourself and your work, until you are able to view it objectively. Then you can more easily find misspellings, plot holes, mangled sentences, and so on.
Read your work aloud; read it backward, too. This will help you find places that might trip up readers, phrasing that doesn’t quite make sense, inconsistencies, etc.
Find a critique partner or group you can trust. Give feedback, and get feedback from others. Encourage one another, but also be honest.
Read! Study your genre. Learn what works, what doesn’t, and why. Also, read outside the genre(s) in which you write. Expand your horizons. The best writers tend to be literary “Renaissance men”: they possess a variety of writing skills—or are willing to acquire new ones—and they have a deep, broad knowledge base.
“Use discernment when receiving contradictory advice, and beware unremitting praise.”
“We are all blind to a certain degree to our own writing.”
Rebecca L. Miller
“If you hear from three or more readers that something isn’t clear—change it.”