What the Senate does
the Senate scrutinises and revises legislation. Members are not tied to a single division, so any of them may read your letter — but they are far more receptive when the subject matches something they work on.
Pick by expertise, not at random
Look for the committees a member sits on, and the subjects they speak on. One well-matched letter to a Senator who already works on your issue is worth twenty scattered ones.
Keep it short and specific
Name the bill, clause or decision. Say what you want them to do — table an amendment, ask a question, or raise it in committee. Then stop. Long letters are read last.