From Consumerist: How To Write To Congress:

Writing to Congress is the single best way to express your view on public policy. The average consumer has a surprising ability to influence legislation by crafting a well written missive. Let’s find out what the common mistakes to avoid are, how the process works, and the best ways to ensure your letter has the greatest impact.

The article continues with specific suggestions on writing your letter, how to format it so it will be read, and where to send it to be read by the right person.