The following was taken from http://usgovinfo.about.com/library/weekly/aa020199.htm

I quote it verbatim here to prevent link rot from losing this great bit of information later.

People who think members of Congress pay little or no attention to constituent mail, are plain wrong. Concise, well thought out personal letters are one of the most effective ways Americans have of influencing law-makers. But, members of Congress get hundreds of letters and emails every day. Whether you choose to use the Postal Service or email, here are some tips that will help your letter have impact.

Think Locally
It's usually best to send letters to the Representative from your local Congressional District or the Senators from your state. Your vote helps elect them -- or not -- and that fact alone carries a lot of weight. It also helps personalize your letter. Sending the same "cookie-cutter" message to every member of Congress may grab attention but rarely much consideration. 

Keep It Simple
Your letter should address a single topic or issue. Typed, one-page letters are best. Many PACs (Political Action Committees) recommend a three-paragraph letter structured like this:

  1. Say why you are writing and who you are. List your "credentials." (If you want a response, you must include your name and address, even when using email.)
  2. Provide more detail. Be factual not emotional. Provide specific rather than general information about how the topic affects you and others. If a certain bill is involved, cite the correct title or number whenever possible.
  3. Close by requesting the action you want taken: a vote for or against a bill, or change in general policy.

The best letters are courteous, to the point, and include specific supporting examples.

Addressing Members of Congress

To Your Senator:

The Honorable (full name)
(Room #) (Name) Senate Office Building
United States Senate
Washington, DC 20510

Dear Senator:

To Your Representative:

The Honorable (full name)
(Room #) (Name) House Office Building
United States House of Representatives
Washington, DC 20515

Dear Representative:

When writing to the Chairperson of a Committee or the Speaker of the House, address them as:

Dear Mr. Chairman
Dear Madam Chairwoman

Dear Mr. Speaker
Dear Madam Speaker

The above addresses should be used in email messages, as well as those sent through the Postal Service.

Finding Their Addresses
Senate and House of Representatives

Email Addresses & Web Sites for Congress is a massive project of the University of Michigan Library Documents Center. The information is priceless.

U.S. Supreme Court
Contact Information - US Supreme Court. The Justices do not have email addresses, but they do read letters from citizens.

Many members of Congress maintain one or more offices in their home states where they can also be contacted. These addresses are typically listed on the members' web sites.

To Conclude
Here are some key things you should always and never do in writing to your elected representatives.

Always

  1. Be courteous and respectful without "gushing."
  2. Clearly and simply state the purpose of your letter. If it's about a certain bill, identify it correctly. If you need help in finding the number of a bill, use the Thomas Legislative Information System.
  3. Say who you are. Anonymous letters go nowhere. Even in email, include your correct name, address, phone number and email address. If you don't include at least your name and address, you will not get a response.
  4. State any professional credentials or personal experience you may have, especially those pertaining to the subject of your letter.
  5. Keep your letter short -- one page is best.
  6. Use specific examples or evidence to support your position.
  7. State what it is you want done or recommend a course of action.
  8. Thank the member for taking the time to read your letter.

Never

  1. Use vulgarity, profanity, or threats. The first two are just plain rude and the third one can get you a visit from the Secret Service. Simply stated, don't let your passion get in the way of making your point,
  2. Fail to include your name and address, even in email letters.
  3. Demand a response.

Identifying Legislation

Cite these legislation identifiers when writing to members of Congress:

House Bills: "H.R._____"
House Resolutions: "H.RES._____"  
House Joint Resolutions: "H.J.RES._____"
Senate Bills: "S._____"
Senate Resolutions: "S.RES._____"
Senate Joint Resolutions: "S.J.RES._____"

Daily Schedule and Agenda of Congress
Updated every day Congress is in session. Also includes votes, bills considered, and other information on the U.S. Congress.