In drafting a demand letter, there are a number of different mistakes that you need to take care to avoid. First and foremost, you should never use any nasty, abusive, or threatening language in this document. A demand letter should always be polite.
Using abusive or threatening language will not work in your favor — not only will it make the defendant less likely to want to settle your claim, it could even cause damage to your actual case.
Beyond that, you should never include dishonest or misleading details in your demand letter. Every line that you put in your demand letter could potentially be used against you. Be careful.