Before writing the letter, make sure that you have the facts straight and that your complaint is legitimate.
Type your letter if possible. Use a spellchecker, or have someone proofread your letter. If it is handwritten, make sure it is neat and easy to read.
Generally keep your letter short and concise. Write short paragraphs, and almost always keep your letter to one page. However, do include all important facts. Include important dates or places (for example, when and where you bought the product or received the service), and include any additional relevant information you can (such as the product number or type of service).
While writing your letter, remain diplomatic and courteous at all times. No matter how justified your complaint may be, do not allow your letter to become angry, sarcastic, or threatening. Keep in mind that the person that reads your letter will often not be the person responsible for the problem.
Try to put yourself in the other person's place, and write your letter accordingly.
Include copies of any documentation relating to your complaint. Do not send original documents.
Keep a copy of the complaint letter for your records.
If your complaint letter does not bring about the results that you hoped for, consider writing another letter with a firmer tone, or try writing to someone higher up in the chain of command.