First you need to pick your battles. People who complain a lot and are constantly unhappy with something become sort of the boy who cried wolf. Complaining about issue after issue makes people see you as just another hassle. If it’s something small you can get around let it roll off or mention it without making a big deal.
When you do find something that really does bother you and is something that you refuse to deal with any longer like poor treatment or rudeness from an employee then a complaint is surely warranted. Then you should gather as many details as you can about the event, the time it took place, the person who did it, where it took place, and any specific details you can add to the letter. The more details you can provide the better the issue can be dealt with.
You should always keep in mind the goal that your complaint is intended for. What change do you want to have made? Be clear and precise as to the changes you think should be made, if not they could change for the worse. You should also make sure the person receiving the complaint has the correct authority to deal with the issue. You would not want a bus boy making sure your food is cooked properly that is the job of the chef.
Be professional and polite, if the complaint is taken as an attack or derogatory you could be perceived as the problem. You catch more bees with honey anyways and usually it is not the fault of the person receiving the complaint so give them a break.
Provide your contact information if you do there will be a better chance that they will follow up with you and you can see that the perceived problem is being dealt with. You yourself can always follow up on the complaint if you feel it is not being dealt with or avoided. If it still has no effect raise the stakes a little more and move up the chain of command to someone with even more authority. Have a second letter and reference the first and explain the inconvenience of having to write a second letter.
Persistence is important and sometimes required since a lot of people think they can throw it under the carpet and it will disappear. You will be the one that benefits from the change so see that it is made.