Once you have recognized these leads, nurture them with follow up through various modes like emails, phone calls, e-newsletters, postal mails, etc.
If you have a company blog, it can be the best place to announce your webinar. Through blogs, you can target your audience by discussing how to register for the webinar and the webinar content itself.
As soon as a person register for your webinar, send them a thank you mail and a confirmation mail. Sending this is not only good manners, but will also help get a confirmation from the registrant side.
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.