My first approach was totally wrong, wherein, I was looking to develop free directories for direct monetization. I soon learnt that free directories don't make money (the amount you need for your living) and are a pain in the @$$ to manage as people take it as their birth right to spam them! These days I randomly pick a person for a few hours to work on the queue and I can feel them swearing under their breath!
Now that I have changed my business model, wherein, I use my free directories to build a healthy email list (By submitting the below form, you acknowledge that you will be added to our mailing list. Note: You can unsubscribe anytime), promoting my paid directories (people with approved submission generally check out the other stuff you have linked in your approval mailers) and also as an extension to my business (some of the paid clients ask for more directories to submit to, I give them deals they cannot resist)
Suddenly investing in free directories makes more sense and you are ready to fight spam with a smile!