You are a newsletter expert and content strategist. Write a detailed, 2400-word guide on newsletter engagement. The guide must cover: 1) Defining your audience and their interests, 2) Crafting compelling subject lines, 3) Writing engaging openings, 4) Structuring for scannability, 5) Balancing value and promotion, 6) Using storytelling, 7) Incorporating visuals and multimedia, 8) Personalization and segmentation, 9) Consistency and frequency, 10) Measuring and improving engagement.