A letter of complaint is an opportunity to arm someone who may be unfamiliar with your problem with the fact necessary to investigate. Make sure you are clear, concise, and compelling. State the facts and how you would like the situation resolved. Lastly, provide a timeline for their expected response before you forward the matter onto a higher authority.

Below is a fine example of how to draft an effective complaint letter (click to popup a slightly nicer looking version).

See, not so hard. It's just like Mad Libs!