Address the issue you right away. In the first paragraph, explain why you are writing the letter. Include only the facts, not your opinion at this point. When you start to give your opinion, that's where you might lose your tactfulness.
Add a compliment. Once you explain the issue, compliment the person or company. For example, a complaint letter about a product might now explain that you have been a loyal customer and are usually very satisfied with the product.
Avoid giving advice and instead, phrase your comments in the form of a question, such as, "Could you please take a look at my proposal?"
Talk about how doing what you propose or what you're asking will be mutually beneficial. For example, when rectifying a complaint with a refund, you are happy because you got your money back and the company benefits by not losing a customer. You will also then tell your friends and family that the company was helpful in resolving your complaint, which can create more loyal customers. Be careful with the wording in this section. Stay positive and polite to remain tactful.
Include an acceptable solution at the end of your letter. This might be receiving a refund, getting something repaired, having a question answered or having your suggestion considered. Use polite, not forceful language and include the word please, such as, "Please consider this suggestion and let me know if there's anything I can do to help implement this new program."