• Be clear and concise. Describe the item or service you bought and the problem. Include serial or model numbers, and the name and location of the seller. If you’re following up on a conversation, be sure to say who you spoke with and confirm the details of your discussion.
• State exactly what you want done and how long you’re willing to wait for a response. Be reasonable.
• Don’t write an angry, sarcastic, or threatening letter. The person reading your letter probably isn’t responsible for the problem, but may be very helpful in resolving it.
• Include copies of relevant documents, like receipts, work orders, and warranties. You also may want to send copies of emails and notes from conversations you’ve had with the seller about the problem. Keep your originals.
• Include your name and contact information. If an account is involved, be sure to include the account number.

via FTC
See also their sample letter: FTC – Sample Consumer Complaint Letter