Sign up with http://Congress.org and they will automatically find your congressional and senate leaders, as well as the president. You can quickly send them a short letter (email or prited) supporting quick action in Syria. The shorter the message the more likely it will be read.
Here is a sample letter:
The situation in Syria is deteriorating quickly. We have to act quickly to prevent further deaths and injury. Assad's regime is mercilessly killing men, women, and children indiscriminately with tanks, aerial bombardment, and armed troops and thugs. A full blown massacre is under way!
We need to pressure the Assad regime and we need to tell them that Assad is illegitimate and must go. Like we did with Gadaffi and Mubarak. Assad has already killed far more people than Gadaffi and Mubarak. the freedom fighters on the ground in Syria also needs to here that we support them!
Thank you and please act quickly.