I sent the following message to both my senators and representative. Feel free to use it as a template for your own message to your congressmen.
Dear Senators Rubio and Nelson, Representative Ros-Lehtinen, and the rest of Congress,
As representatives of the American people, your duty is to carry forth the will of the people in setting the policy of the United States. While I believe that the President is incorrect when he states he has the authority to intervene in Syria unilaterally, he has been forced politically to ask Congress for support. It is now up to you to determine what is the best policy going forward.
I would urge you to note the vast and sad history of American interventions in the Middle East, along with the many abandoned promises of the executive branch, which has virtually always pledged to keep attacks short and decisive. The atrocities occurring in Syria are horrific, but there is no simple path to an outcome the United States would benefit from, and there is little evidence that America can miraculously make the situation better using military force.
As a member of Congress, your duty is not to solve complex Middle Eastern civil wars, where no side is the friend of liberty and democracy; your duty is to the American people, where we are already stretched thin on budget challenges. I urge you to take a stand against continued overseas involvement, and use the power of the legislature to keep the executive branch in check. The President may not feel he can stay on the sidelines while horrible events happen in the world, but that is not his call to make.