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.
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:
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
To Your Representative:
The Honorable (full name)
(Room #) (Name) House Office Building
United States House of Representatives
Washington, DC 20515
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.
Here are some key things you should always and never do in writing to your elected representatives.
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._____"
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.