For SEO, content is king. Consider publishing e.g. a unique article every week on your directory under an article section or something. Google will love you for this and if you write about directories you could bring in some extra traffic from google.
For keywords, if you were deciding to go with a niche directory then they will be obvious. As for a general directory it is very competitive. It is possible to get there but it may be more worthwhile to focus on forum posting etc.
- social networking? e.g. facebook, twitter
- if you decide to write articles as I mentioned above then submit them to digg and other sites
- make a youtube video to do with directories.
- word of mouth (e.g. give something out for free)
- buying paid links on webmaster related sites e.g. backlinkwatch.com
I hope some of this was helpful and I strongly encourage you to go into a niche directory.