Follow the submission guidelines
This might seem a little obvious, but it's honestly very important. Guidelines vary widely, so double check that you're including all the details asked for.
Give concrete examples of your writing credits
Rather than saying things like 'I was born to write', include facts or examples. Like, 'I've had short stories published online', 'I won a local writing competition' or 'this is my second full length manuscript'.
Be canny about including information
It's great to share things you've done to develop your writing, including relevant courses or training, but don't overload your letter with everything you've ever done. Pick one to two to focus on.
Personalise your message
You may be sending a cover letter to many different people, but never, ever send a group email. You are asking someone to take the time to seriously consider your work so it is important to personalise your email or letter.
Focus on the positives
Getting published can be a long and sometimes difficult road and it can be tempting to mention how hard it's been for you. This is not the place to do it. Don't disparage any other publishing professionals and keep your letter as positive as possible.
Keep it simple
It can be tempting to try and get your work noticed with the help of a gimmick or gift along with your cover letter, but agents and publishers are looking for someone with work that speaks for itself.
Looking for some more advice? Writers & Artistshave a variety of guides and information about submitting to agent and publishers. For advice on funding proposals and applications, the Cultural Enterprises Office has some handy resources.