First of all, there is no question that professional letterhead is important. It need not be expensive, in fact, nice looking letterhead is relatively inexpensive.
Secondly, a letter must be properly formatted. This is easy to learn as there are many places where one can see what a proper letter format is such as on the Internet.
It is also extremely important that a letter is grammatically correct and free of misspellings.
And, a letter should be to the point without exaggeration or tangents.
A letter should always be civil and polite. Even if you are upset about something, you can say what needs to be said without being offensive or rude. This is especially important, particularly since it seems to be a custom in this age to be as obnoxious as possible, something that can be seen almost everywhere, the sort of thing that has resulted in road rage. For example, right here at this site, management has stated in very clear terms that any discourse must be polite and civil. It’s too bad they have to say it, but that’s the nature of our age, the age of rudeness, I call it.
I think if more people collected their thoughts and sat down and wrote a cogent letter they would feel much better about things in general and there is nothing like having your letter published in the local paper to make you believe you are actually impacting the world in some small way.
I recall several years ago, a friend of mine told me my name came up at the Christmas party he went to. When I inquired how that could happen, he conveyed to me that a discussion about letters to the editor came up and so did my name, one person stating how she looked forward to seeing my letters because they always had an interesting point of view. This is perhaps the secret to getting letters published, expressing a view that no one has before, something I make a point of doing as often as possible.
Another person I ran into one day when he learned my name said to me that he had been reading my letters for years in the local paper and wondered who I was. He said he liked my letters because they were brief and to the point and always stated their position strongly. This is another secret to successful letter writing. Believe in what you have to say and say it as directly as possible without emotion. As Mario Puzo said, just tell the story.
And that is the secret of good writing.