For most people, disappointment is a part of life. The feeling is so commonplace that many people swallow the emotion without complaint, slowly feeding a growing pool of resentment. They might make excuses for the offending party, telling themselves "They are doing the best they can." That may be true, but a little constructive criticism never hurt anyone. Businesses and other entities that receive letters of disappointment soon realise it is in their best interests to make room for improvement. As the saying goes, "The squeaky wheel gets the grease."
Organise your facts. The person or company you're writing the letter of disappointment to cannot improve their business or personal practices if you don't clearly explain how they have failed you. Before you write the letter, have a list of instances where you felt disappointed, organised in chronological order. If you have official documentation from other sources, also have those handy before you begin your letter.
List your grievances. Explain clearly what your expectations were and how they were not met. Use documentation from other sources if necessary.
Explain how the grievances made you feel. For example, if you're writing to a fast food company about repeatedly rude customer service, explain that it makes you feel as if your money and patronage is not valued within their organisation.
Be kind. Remember that the goal of your letter is to help a person or organisation improve, not degrade them. You are simply pointing out ways in which you feel they could perform better. When writing a letter of disappointment to a company, remember that the person reading the letter will likely not be the person with whom you have the grievance. Don't be rude or nasty; keep the tone of the letter kind, or at the very least, professional.
Make suggestions for improvement. Now that you have pointed out the problem, assist by pointing them toward solutions.
Present your demands. If you feel you deserve restitution for your experience, a letter of disappointment is the place to ask. Whether you're asking for a direct refund or a discounted or free service in the future, indicate that meeting your demands will go a long way toward keeping you as a customer.
Type the letter. Professional presentation of your disappointment will let the recipient know that you are serious.
Make copies of the letter. In case your letter is lost, becomes an issue of dispute, or becomes the first in a long chain of disappointment letters, you should have a copy for your records. If you are including other documents with your letter, make copies of those documents and send the copies. Always keep original documents for yourself.