The best cover letter in the world is not really going to help you sell your story. Your story is going to sink or swim on its own.
However, a bad cover letter is perhaps more likely to undermine your chances. It can clue the editor into the fact that you’re new and inexperienced or, worse yet, that you’ve settled for being published in mediocre markets.
The cover letter will rarely sway things either way. Some editors ignore cover lettersr; they read the story first. But not all editors do that. Why not give yourself that extra edge?
Make sure that you avoid these common mistakes:
1) Make sure that your formatting is correct.
2) Always send it to a person and make sure that your initial contact is more formal (versus more casual). LinkedIn is your friend here for finding the person's name and title.
3) Write only what is relevant and necessary. Business writing is concise and action-oriented.
4) Follow any stipulated guidelines.
5) Don't summarize your story in the letter. Let your story speak for itself.
6) Include only writing-related accomplishments in your cover letter. It's also okay to say that you are a new writer. Editors are always looking for new, undiscovered talent. List your top 3-5 accomplishments and not a laundry list.
7) Make sure that your cover letter is short and tailored to that editor and his/her company. Include the story title, genre, length, and any relevant awards. Use your website if available.