If necessary, provide a few testimonials that relate to your argument. You know, sometimes in order to keep one's community quiet and peaceful one has to take action.
Last week they released their recommendations on how best to combat that problem. If you think that your argument is relevant and correct, then put your best foot forward.
Avoid long, crammed sentences and paragraphs because they are intimidating and hard to digest.
Just remember the famous quote by Mark Twain - "I didn't have time to write a short letter, so I wrote a long one instead". Once you know the purpose of writing a persuasive letter, your next step will be to know your reader.
Second step Do you know your reader well?