If you can't afford or choose not to pay for the services of a professional, get as many people to proofread your manuscript as possible. The reason? Having others read over your manuscript will reveal the blind spots you are oblivious to in your own self-editing.
How do these blind spots develop? It's very simple. Writers work with their manuscripts so intimately and intensely that, after a while, they read the text in their heads rather than the text on the page. To this day, I am blind to some of the of the errors in my manuscript and only become aware of them when someone else points them out. They are usually small errors and typos like missing or repeated words, but they are errors all the same.
Don't deny the existence of blind spots - get as many proofreaders as you can before you place your book on the market.