Search directories differ from search engines by providing a range of categories for users to browse. Rather than enter a keyword into a search box, users click through categories and sub-categories narrowing down their options.
You could say that search engines are like going straight up to the sales assistant and asking what they have in evening wear; search directories are like browsing through the store and seeing what catches the eye.
How you make your site catch the eye in a directory is actually pretty similar to standing out in a search engine. It’s all about relevancy — a mixture of keywords and links.