letters
First you must find out how to contact your elected representative, weather they are at the local, state, or federal level. At the United States Government website, you can find the contact information of your state Legislators, Governors, U.S. Representatives, Senators, and the President. Access it here
It is also important to address the letter correctly, use their full name including their title(s). Write, "Dear Mr. President Barack Obama," not, "Dear Barack."
In general, letters should be short and concise; three short paragraphs should be plenty. These are busy people after all. The first paragraph should state that you support or oppose a position or piece of legislation. The second paragraph should explain, briefly, the reasons for your support or opposition. The third paragraph should ask the Congressman to write back explaining his position on the legislation. Avoid exaggeration and, when appropriate, document your position with an accompanying article or editorial.
State your view firmly, and with conviction, but remain respectful. Avoid insults or making threats. It is impertinent to be reasonable, factual, and friendly. Even if you disagree with your elected official on most issues, be sure to maintain a tone of respect. It will establish that you are fair and will encourage them to pay closer attention to how you view their performance as an elected official