Its good to see that folks aren't looking at democracy as a spectator sport. Here are a couple of things think about when contacting your representatives.
1) However you send it, your message will be received and tallied by an overworked staffer who is also likely to be a partisan of whichever party your representative is from. Be courteous and get to the point quickly.
2) You don't need to structure an impassioned, detailed argument for your position. You should also be wary of burying your position deep in the text. Stating your position in the subject line or first sentence or two is all that's needed, they'll probably just quit reading there and put it in a pile of other letters for or against the legislation.
3) Different methods of communication carry different weights, which relate to the amount of effort it takes to send the message:
A personal, original letter directly from you, sent via US Mail or faxed if the vote's soon is probably the most powerful.
A phone call is probably second.
Canned messages that are pre-prepared, or messages sent via advocacy group websites where all you have to do is enter some personal info (which gets you on the group's mailing list) and click your mouse probably carry less weight than phone calls or letters. Think about how much attention you pay to spam messages that are broadcast by the bushel.
4) If you can reference the bill number or name the specific piece of legislation it will make things very clear which way you want them to vote. Saying "Please protect the environment" is not as easy for a staffer to relate to specific legislation as, "Please vote no on House Bill No. ####, The Blah Blah Blah Act of 2005."
The activist websites are great places to find out who you need to contact, get the bill number and legislation names, and an understanding of the issue. You can then either copy and paste the language into a personal letter you can sign and send, or use the info when making a phone call to your representative's office.
Keep those calls and letters going to your representatives and senators!
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse