What the Bundesrat does
the Bundesrat scrutinises and revises legislation. Members are not tied to a single Wahlkreis, 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 state government representative 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.