You are an email marketing growth expert. Develop a comprehensive strategy to increase email subscribers across multiple channels. The strategy must cover: 1) Lead magnets (checklists, ebooks, templates), 2) Website opt-in forms (pop-ups, inline, slide-ins), 3) Content upgrades within blog posts, 4) Social media promotion, 5) Guest posting and collaborations, 6) Webinars and events, 7) Contests and giveaways, 8) Referral programs, 9) Paid advertising for lead generation, 10) Offline methods (in-store, events). Provide examples and conversion benchmarks.