The first thing to make sure is that your complaint is genuine by yourself. There is no point in shooting a complaint letter for silly reasons.

To write a good complaint letter, first gather facts and figures related to the issue you are facing. Describe in summary your bad experience with the product you bought or service rendered to you. If possible arrange the events in a chronological order. If you are complaining about a product you bought some months ago, you can use the available supporting details such as date of purchase, date first serviced, date on which the first problem first reported etc. are some of the valuable details you can provide which will help the company to find out the root cause of the issue.

The core content of a complaint letter should be the issue you are facing. Draft the complaint letter by making the issue center point. Rest of the details you provide should complement the details you are presenting as the issue. There is no point in dragging the complaint letter to unnecessary details, as it will only help to distract the attention from the main issue you want to put forward. Make sure that you provide only relevant and valid points in your letter. Good complaint letters often has immediate result. Never use words, which is provocative as it do nothing good but create a bad impression about you.

If you have any additional documents to provide, such as invoice, service bill etc. please number it and mention them as enclosures. These documents help speedy resolution of the issue as they help the company to cross-verify the details submitted by you. They help to track down the issue to the point where it is found.