You may be angry, and may have every right to be, but being rude will only put the recipient on the defensive. Write in a respectful tone and avoid making threatening, angry or sarcastic comments at all costs. Remember that the person reading your letter was not directly responsible for whatever happened, and they will be much more responsive and willing to please a gracious, polite customer than an angry, accusatory one.
• Remember, the company to which you are writing probably isn't out to get you, intentionally. Most companies have an interest in their customer's satisfaction.
• You will have much better success treating the recipient as somebody who wants to help you, rather than assuming they're filled with malicious intent.
• Don't write when you're furious. Wait to write your letter until you have calmed down. Or if you wish, write the letter while you are fully steamed, and then let it sit for a day or two before you send it. In all likelihood, you will want to rephrase things in a less-incendiary way.