Before you send your manuscript to an agent, it is important that you allow others to read and critique your story. And by others, I don’t mean your mother, your spouse or anyone else who loves you and wants to see you succeed. The person or person who read your novel should be objective, familiar with the genre and the essentials of good writing. They should also not be afraid to hurt your feelings and rip your manuscript apart if necessary.
There are many online critique websites at your disposal. You have only to complete an internet search and a little due diligence to find one that works for you. If formalized critique websites are not your style, try using social media formats like Twitter or Facebook to find readers of your genre interested in critiquing your story.
Just as the best meals can be ruined by too many proverbial cooks in the kitchen, a well-written novel will suffer from too many critiques. If you let ten people read your story, you will have ten sets of critiques to wade through. Of course, if all ten readers point out the same flaw, obviously you should address it. But it is better to find one to two dedicated readers, whose judgment you trust to offer suggestions.
Your critique partners do not have to be writers but they do have to be readers, able to recognize grammatical, character development and plotting errors at the minimum. It is always surprising to discover what flaws other readers can find in your work-in-progress. But once found, your writing will only improve, as well as your chances of having an agent represent your manuscript.