Let me preface by saying this article was written for
phpLD directory owners, but it is definitely something anyone can use.
Whether your directory is a niche or not, my advice would be to find a "niche" on the web where there is an audience you can promote to. Just to give you an example, maybe you decide to become really active in a forum like
Work at Home Mom (don't have to be a mom or even female). You have a signature link. You make friends with some members. You maybe...