Even though you're dying to call Jesse Jackson a preachy parasite, stifle the urge. Your letter should be logically organized. First a brief recitation of the argument you are opposing, followed by a statement of your own position. Then present your evidence. Use facts, figures and expert testimony whenever possible.
Readers respect the opinions of people with special knowledge or expertise.
Proofread your letter carefully for errors in spelling, punctuation and grammar. Read your letter to a friend, for objective input. One suggestion is that a letter shouldn't be mailed the same day it is written. Write, proofread and edit the piece. Then put it aside until the next day.