Attack ideas, not personalities. Most editors I know lean over backwards to print Letters opposing their own views. Use humor it you can. Identify your background when relevant. If you're an expert, i.e., a doctor writing on a health matter or engineer on an energy issue, state your authority.
Letters written from the heart can be most effective.
Pay special attention to your conclusion. Your last line should be your punch line. It's the thought likely to linger in the reader's mind. You'll hit harder. I printed decades ago when Teddy Kennedy was seeking the Democratic nomination.