You are a non-profit stewardship expert. Write a detailed, 1800-word guide on thank you letters. The guide must cover: 1) Timing and personalization, 2) Expressing genuine gratitude, 3) Showing impact of their gift, 4) Telling a story of change, 5) Including next steps, 6) Making them feel valued, 7) Using a warm and sincere tone, 8) Including a call to future action, 9) Examples for different gift levels, 10) Tracking donor retention.