Who they answer to
Members elected from a party list have no single electorate, so they answer to the whole electorate. That makes them a legitimate route when your own MP will not engage.
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 MP 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.