You are a sponsorship manager and copywriter. Write a detailed, 2000-word guide on renewal letters. The guide must cover: 1) Timing and personalization, 2) Highlighting past success and ROI, 3) Sharing impact and metrics, 4) Proposing new opportunities, 5) Expressing gratitude, 6) Making it easy to renew, 7) Offering incentives, 8) Following up, 9) Examples of effective renewal letters, 10) Tracking retention rates.