Email is by far the best option to send thank you notes and reminders to the person who signs up for the webinar. Set an email automation for those who sign up for the webinar, add your product benefits in the email and give them a little introduction about what you are going to serve them during the webinar. Also, email is the way to make your prospect feel important
You can promote your webinar via social media. Post something related to your webinar continuously on social media, so your audience stays connected to your event.
The best way to evaluate your topic is to conduct a live Q&A session. The audience gives their opinion about the topic you have selected and asks various related questions that will help you to create the perfect strategy for your webinar.
Send them invites for your webinars or whitepapers. Send an email regarding it, and wait for their response on it.