If your company has collected contact information from potential and current customers, you can use this list when publicizing your webinar. Send out emails to everyone on your list to announce your event. In some cases, you may wish to send out more than one notice, such as two weeks before, one week before and even the day before the event. In most cases, three well-spaced emails are the most effective.
Furthermore, make sure the email invitations sent to your mailing lists are well-formed and have been tested internally, before being sent out. It is suggested that you test your mailing lists by sending out both text-only emails and HTML emails, and see how those on your list respond. Often times HTML is preferred for B2B oriented emails, as it is often times more attractive and professional-looking than a standard html email. In contrast, audiences attending a webinar for a B2C type subject seem to respond better to personalized text-only messages from people they know.
Regardless of the type of webinar/webcast you are hosting, make sure to take advantage of the power of mailing lists for your future events!