I have talked repeatedly about the importance of making sure that your story is complete and ready for the eyes of any editor, agent or publisher that may view it. It is vital for success that you present a copy of your work that is clean, neat, follows guidelines and has proper punctuation and style. Many will emphasize grammar here as well and while having a basic knowledge is key, I believe that in creative writing there are some rules that are fine to bend if not break, just don't do it too often.
We have talked about proofreading and even having other help you with this process. Once the book is complete and you are ready to find your agent or publisher you then craft the perfect query letter. Once you write the letter though there is still a very important step you need to remember. You have to edit your query letter. Proofreading is not exclusive to the story itself. In fact it is almost more important to proofread and go over all of the details for your letter than for the book.
I few small typos in a fifty thousand word novel can be forgiven more easily than misspelling a name or location in your one page query letter. Did you double check the spelling of the agent's name? Is the address correct? What about the about the author section? Did you ramble on about the blue ribbon from junior high for your investigative reporting on school lunch meat or possibly gloss of the section trying to fill space with a run on sentence that ultimately says nothing about your experience?
This is your first impression for whomever you have sent your book to be considered. It is possible in time to overcome a negative first impression but in the publishing industry you rarely get that desperately needed second chance. You have to make sure that you come across as interesting, knowledgable and professional with a great story they are going to just have to request. Once the book is done you are no longer a writer, you are a salesman.
Proofing your query is sort of like practicing your sales pitch. In fact that is exactly what you are doing. You are going over every little detail, checking to make sure you have gotten all of the information right and putting that little sparkle into it so that when the agent gets their hands on your letter they see everything you as an author and your story as a product have to offer them. It takes time to get it down but no matter how many letters you have sent or books you have written, following the steps and remembering to always double check your work can be a big key to moving to the next level.