I think it is pretty much the same as with any other site. The advantage with a directory is that if you require a link back to your site it will grow your PR and this is a great advantage with a directory
Get traffic to your directory is quite simple. I setup my directory few months ago, add link in my sig here and I get first submitions. Than I open thread about my directory in Directory section of these forum. Few members submited their sites and I approved them fast. They post reply in my thread they like my dir and other users submit their sites. I waited few weeks and directory lists have added my directory. Now I have PR2 and get about 100 unique visitors every day.
In my opinion, there are too many directories in the market now, even niche one. Marketing is something generic even a niche one so it's hard to get some attraction for a real people (visitor that need real information, not webmaster looking for submit their sites to increase their one-way link popularity. Most of real visitors use another services like search engine, even technorati for blogs. As bess mentioned above about advantage of directory is easier to get link back to your directory just asking people who submit their site to give link back but that is only advantage in seo terms, even you can get a little of traffics. If your aim is to sell links on your directory, so you target is webmasters and you may consider to offer free submit to increase contents and offer a cheap ($1-$5) for featured listing to make some bucks. If you want to sell link at high price, the patient is the key because it takes a lot of time to built link popularity and contents much enough to attract search engine to think if your directory is good and have authoritative. The directory value it takes too much time to built and I don't think it's worth. You may use basic marketing for your directory 1. Keep submitting to another free general directories, both new and old to get links as many as possible. 2. Buy some permanent link from much older directories (ex. joeant ) to make your site some value. 3. Get your directory reviewed by another well-known person, shoemoney or john chow, but they may not accept to review your site if your site too generic or do not have any interesting for them, up to them. 4. Keep posting in all famous webmaster forums (like here) in trying to help another people, not spammy their threads. You will get traffics much more than banners or text link from another site, most of visitor do not interest on ads. 5. Write some useful articles and put them on your directory to make it unique and put another to submit article directories to get backlink and some traffics, you may consider to create content that unrelate to your directory but attractive for people to read, find out who are you and go to your site by their own.
email the creators of the directory autosubmitter programs and ask that your directory be included in the next software update. Also you can do the same thing with an article directory.
I think if you start with a free directory, keep it as free and keep promoting it. This way many sites and links, already linking to good free web directories will add your links which can help you give a PR boost.