Submit Articles & start a blog. If it's a niche directory you'll still probably manage to get listed in lots of other directories. Apart from that all the usual stuff - social bookmarking, social networking in your segment, other forms of viral marketing blah blah blah
Directorys Articles And for directorys Directory lists and directory of directory bring nice traffic + webmaster forums
Consider to get listed in the strongest link directories with high PR and also go for promotion through blogging, online Forums and social media.
Start link builging for your directory and also submit your directoty into other quality directories. Keep adding quality sites into your directory regularly (visit categories of Dmoz and add the listed sites into your respective categories yourself) to keep it live and you will see the difference.