....one e-mail at a time.
As small as it may seem, writing your elected officials can have major impact, if done properly. It doesn't matter if you are writing FOR or AGAINST some piece of legislation; your voice is heard, even if they choose to vote otherwise. Members of Congress do weigh the opinions of their constituents, in an informal vote.
And here's a fairly simple way of doing it; every elected official has a "contact me" page, of which many are linked on our side bar. But one site (Congress.org) puts it all together as one message, if you want it really simplified. Check this out:
Contact Your Federal Officials
Contact Your State Officials
You'll end up getting their e-mails (from which you can unsubscribe), but it's a fairly simple way to contact your Congressional delegation. If you need to contact just one, go to their "dot gov" page for that.