Hi Kelly,
We have found that HTML emails, if they follow best practices, can be very successful in promoting webinars. To Kim's point, keeping a simple design and a clear message is key... For an interesting analysis of an email that fell short, you can go here: http://spearmarketing.com/blog/design-choices-more-sink-webinar-invitation/
Also, with regards to invitation frequency, definately try to not exhaust your database with too many invitations to the same event. Some of our clients will send the initial invitation and then a "reminder" which is customized to look like a "FWD" from their lead owner... if you use tokens correctly, this can be very effective.
Julie