You are an advocacy and campaign copywriter. Write a detailed, 2000-word post on advocacy letters. The post must cover: 1) Crafting a compelling subject line, 2) Stating the issue clearly, 3) Making a specific ask, 4) Using emotional appeal, 5) Including facts and data, 6) Personalizing for the recipient, 7) Making it easy to take action, 8) Following up, 9) Examples for different issues, 10) Measuring impact.