Did not make this money by throwing up a directory on expired domains and flipping. Pure directory submissions.
The directory business is not about SEOing a directory your way to the top of the SERP's for the term "web directory". It's about effective promotion and customer service.
I promote my directories with 125x125 banner ads, email templates, and good customer service. An important element in good customer service is fast review and listng. When I review and list a quality submission within an hour, I often get additional submissions from that customer.
If you own a directory and are looking for ideas of how to promote it, feel free to PM me.