State the argument you're responding to, as briefly as possible, in the letter's introduction. Pick an issue of particular importance to you - don't be afraid to let some passion show through. Stick to a single subject. Deal with one issue per letter. Don't be abusive.
Editors tend to discard letters containing personal attacks.
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. Close with a short restatement of your position or a pithy comment. Use facts, figures and expert testimony whenever possible.