Even an editor needs an editor. I've been known to use too many passive verbs. Writers get so close to their work that it's easy to overlook a typo or misspelling not caught by spell-check. If we get lost in thought, a run-on sentence can result. And, where do those pesky commas actually belong?
Anyone who aspires to be a published author should read the submission guidelines of several different publications. Who is your audience? Is your subject timely? Does your work meet the standards of your chosen publication? Then, hire an editor.
There are different types of editors, so consider which type you require:
Substantive/Conceptual: For fiction or non-fiction, but especially for novels, this type of editor is consulted first. A substantive editor critiques your writing with emphasis on content, technique, organization, and presentation of the complete text.
Copy/Line: Consult this editor next to do a line-by-line proof. A copy editor looks for typos, misspellings, punctuation and grammatical errors, structure, flow, and consistency of text. If extensive rewrites are necessary, repeat this process prior to submission.
Proofread: Prior to final printing, a proofreader reviews the galley for typos, misspellings, and punctuation, grammatical and semantic errors.
An editor may also be able to tighten your story by eliminating the extraneous--keeping in mind that an editor does not select or impel. An editor can only suggest. The story belongs to the writer.
Once an editor is selected, a Standard Editorial Agreement (contract) is advisable for both client and editor that specifies editorial tasks, deadline(s), method of payment, and any special requests.
You know what is said about the attorney who represents himself. The writer who edits her own work is just as foolish.