Send them invites for your webinars or whitepapers. Send an email regarding it, and wait for their response on it.
Take time to craft your marketing copies for your webinar. This copy can also become part of your landing page and other marketing tools. The title and content of your webinar copy should describe each and every detail about the webinar, like what the attendees would gain from this webinar, etc.
While evaluating your webinar strategies, make sure that you take a fresh look at your approach for your webinar, and compare it with your old assumptions to ensure that you are going in the right direction, and it is advantageous to your webinar.