Congress critters do rely on your vote, and their staffers do look at the mail that comes in and take action if they can - and if your letter is compelling enough. To get action, Political Action Committees recommend writing a courteous three-paragraph letter structured like this:
1) Why I'm writing. Who I am. Credentials. Name and address for response.
2) Provide factual detail, not just emotion. Be specific, not general when it comes to describing how this affects you and others. If there's a bill, cite the correct title or bill number. (Thomas Legislative Information System can help you find that.)
3) Close by requesting the action you want (a vote for or against a bill, intervention, or a change in policy).