Choose good quality stationery in white or a neutral shade. Set one-inch margins on all sides, confine your single-spaced content to one sheet of paper, and select an easy to read 12 pt. font such as Courier New, Bookman or Times New Roman.
Address your cover letter to the appropriate party that handles your type of submission. At publishing houses, there are often different editors for fiction, nonfiction and various genres. Magazines--depending on their size--divide the work amongst section heads and editorial assistants. If a name isn't listed in the submission guidelines, it's permissible to call and ask. This will also ensure you get the correct spelling and title.
Provide a short subject line so the recipient will know what your letter is about. In most cases, this will include the words, "Manuscript for Consideration". Using your project's title for the subject line isn't advisable if it's your first correspondence with an editor.
Example: I Know Who You Slept With And I'm Telling.
Cut to the chase in your first paragraph by requesting consideration of your recently completed manuscript, "Such and Such". Identify the genre, its approximate length, your target demographic, and why this book either addresses a current need/interest of the readership or fits the publisher's existing bill of fare.
Provide a brief summary in the second paragraph of what your manuscript is about. For fiction submissions, this will be a teaser that divulges the main characters' names, the central conflict, and the story's setting. For nonfiction manuscripts, reference the titles of some of your chapters and whether there are interviews, case studies, photographs, etc. For magazine submissions,use the second paragraph to discuss the unique slant and timeliness of your article.
Share your credentials in the third paragraph along with reference to any awards you have won for your writing. For book manuscripts, this paragraph should also be used to explain how you plan to help market and promote the book through lectures, blogs, interviews, workshops and so forth.
Include your complete contact information below your signature. Provide a self-addressed, stamped envelope. If you've included the manuscript, be sure to either provide sufficient postage or indicate at the bottom of your letter that the material need not be returned.