Organise an online expert event.
Similar to the idea of interviewing an expert, but en masse. Organise a whole range of experts to speak on given subjects and offer access to the calls as a live event, or offer the recordings afterwards, or both. Getting all of the experts to promote the event will increase your mailing list as they will have to go to your web site to sign up to receive dial in access details.
For more information about how to market your virtual assistant business, take a look at the VA Success Groups Marketing for Virtual Assistants Online Course