There are several ways to collect custom field data: 1. Signup Forms: When new subscribers sign up for your email list, you can ask for additional information beyond the basic email address. 2. Surveys: Periodically send out surveys to gather more information about your existing subscribers. 3. Purchase Data: Integrate your e-commerce platform with your email marketing software to automatically capture purchase behavior and other relevant data. 4. Engagement Metrics: Track how subscribers interact with your emails to gather insights on their preferences.