Interview some experts. If you can talk on the telephone, you can interview an expert in a similar field. You can offer it as a live teleseminar, or group call, or offer it afterwards as a recording. By asking your expert to also promote the interview to their mailing list, you will end up with many of their contacts subscribing to yours.
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