2. Don't include any information that you wouldn't include in a letter to a bank manager. In other words, nothing about your personal life, your cats, your dog, your diet, your beliefs. It's a business letter. Be business like.
3. Don't rubbish publishing. Don't say it's a clique run by idiots who wouldn't know a good book if they saw one, even if that's what you believe. You want to be part of that world, don't you?
4. Don't make any spelling or grammatical mistakes, or typos, or use incorrect names. Is it Sarah or Sara? If you don't know, play it safe and check. Ditto Mrs, Miss and Ms. Some people feel very strongly about their title, so either check or leave the title out. And while you're about it, don't present your letter and manuscript in any other way except the conventional one. This is not the time to get creative.
5. Don't say your mother/husband/wife/children/grandchildren/best friend loved it. It's not relevant. Unless they're the head book buyer for Tesco.
5 things to put in a letter to an agent tomorrow.