To measure the effectiveness of different webinar follow-up email strategies over time, it's essential to consistently track key performance indicators (KPIs). These includ open rates, click-through rates, conversion rates, and attendee feedback. This involves not only comparing the results of A/B tests for individual campaigns but also analyzing these metrics across multiple campaigns to identify trends and patterns that indicate successful strategies. Regularly updating your approach based on data-driven insights will refine your email campaigns for better engagement and conversions.