Every day the news offers us all too many topics on which to comment. State the argument you're rebutting or responding to, as briefly as possible, in the letter's introduction. Stick to a single subject. Deal with one issue per letter. Don't be shrill or abusive.
Editors tend to discard letters containing personal attacks.
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.