House.gov, the official House of Congress website, provides a list of representative pages with contact information. Enter your zip code in the box at the top of the page to find your representative, or click the Representatives link for a listing of all House of Congress members. On each representative's page, look for a Contact link, and keep in mind that most require individuals to enter their zip code, to prove that they do in fact live in that district, before being allowed to use the contact form.
Other methods of finding mailing addresses and phone numbers include third-party websites such as ContactingtheContress.org and CongressMerge.com. On each website, click your state for a list of representatives and senators, and then click the desired representative for full contact information.
Written letters and phone calls generally have more impact than simply using online contact forms or email, states About.com. Note as well that representatives have both Washington, DC and district addresses. District office staff will also talk to constituents in person about many issues.
When writing a letter to a representative, be sure to include your return address and a phone number as well. Try to avoid boilerplate messages, advises About.com. Representatives, and their staff, are more likely to respond to a letter that is not copied from elsewhere. Be as brief as possible while making your point.