Stick to one subject, and try to limit your letters to under three or four paragraphs in length. The shorter the letter, the better its chances of being printed. Newspapers, at their core, are community entities. Editors will be much more likely to publish a letter, and the letter will have much more impact, if it demonstrates local relevance. Use local statistics.
Nothing is more persuasive than a threat to one's home.
Use personal stories. Personal stories are one of the best ways humans have to relate to each other. If you have one that's relevant, use it. If a letter to the editor mentions a Representative or Senator's name, they will see it. It's a good place to ask officials for support and to ask fellow readers to write your representatives.