You can optimize web directory by leaving your site/blog url there with article submission/comments etc. On that way you will get huge backlinks to your site/blog, effective for SEO ranking.
If you are talking about your own web directory promotion then you can promote this with social activities, like facebook, twitter promotion. Also you can exchange traffic with other directory owners.
A web directory is nothing but a collection of website that allow webmaster to submit the site.A web directory is a directory on the www.A web directory organise website by subject and is usually maintained by human instead by software.