US citizens: The American Library Association has created an action alert to send a message to your representative in support of the NIH policy and opposing the Conyers bill. It only works if your representative is a member of the House Judiciary Committee. (If you don't know whether your representative is a member, you needn't look it up; once you enter your 9 digit zip code in the form, it will tell you.)
If your representative is not a member of the Judiciary Committee, it would still help to send him or her a message. The Alliance for Taxpayer Access has drafted a sample letter, which you can revise for maximum impact.
If you have the time, it would help even more to send a message to each member of the Judiciary Committee, not just your own representative. Charles Bailey has collected the contact information for each member.
The open access movement:
Putting peer-reviewed scientific and scholarly literature
on the internet. Making it available free of charge and
free of most copyright and licensing restrictions.
Removing the barriers to serious research.
I recommend the OA tracking project (OATP) as the best way to stay on top of new OA developments. You can read the OATP feed on a blog-like web page or subscribe to it by RSS, email, or Twitter. You can also help build the feed by tagging new developments you encounter.