Send reminder mails to registrants in order to remind them of the webinar. It will allow them to schedule their work accordingly and would help you get the maximum number of people for your webinar.
Be sure to allow time at the end of the webinar for taking questions that your audience can submit throughout the webinar. The Q&A provides a great opportunity to engage your audience and give additional insight to what you have previously discussed, as well as related topics that are of interest to the audience.
You can involve sponsors in your webinars to make money from sponsored ads and announcements which run in between of your webinars. It can be very beneficial when you are attracting larger audiences through your webinars.
It is really difficult to get an audience to be a part of your webinar. You always need to look for a topic from your niche that is trending to be able to capture the attention of a large audience. Do some research and look for customer problems. Be relevant and informative with your webinar topic to encourage more people to attend your webinar.