I think there is a thread on this subject but I can find it

1. decide what software you want to use, some of of the popular directory software: PHPLD, eSyndicat
2. Must direcory software will come with database dump of pre creaed categories, its a BAD idea to use this as your directory will be exactly like thousands of other directories
3. Have a good advert budget or link exchange, directory industry is very competitive
4. Offer free inclusion to draw submission and charge for premium listing
I am sure other will add more to this list