Send an e-mail. With e-mail, you’ll always have a copy of the letter as reference. But don’t write one of those “letters that drone.” Keep the complaint concise, and include vital information such as flight numbers and the names of employees to whom you have spoken.
Keep cool. Most companies “keep tabs on chronic complainers,” so don’t aimlessly ramble or exaggerate. Rude threats are a waste of time. If your anger persists, try putting the letter aside for a day to regain composure. If the company fails to reply within 60 days, try again and include a copy of the original complaint.
Be real. “You’re not going to get two free first-class tickets to Hawaii because your luggage was delayed.” In fact, if you want to get any results at all, you’ll need receipts, resourcefulness, and lots of patience.
Source: National Geographic Traveler