Be as specific as possible, and provide examples that support your assertions. If you didn't include your name and address at the top of the letter, put them under signature. Once you've drafted your letter, leave it alone for a while. Then come back and read it through from beginning to end. If possible, trim your letter to a single page.
Longer letters are likely just to get skimmed, rather than read carefully.
If you've written an angry letter, edit it especially carefully. Stating that you're furious is one thing. Launching into a diatribe or insulting the character of the person to whom you're writing, however, greatly reduces your power to persuade. On the other hand, you may find in rereading it that your letter doesn't state your feelings strongly enough. Don't hesitate to add the adjectives that will convey that you're really enthusiastic or deeply troubled about the issue.