Repeating a few but here are a couple ideas:
- Forum sig links
- Create nice logos & banners that are brandable and use them whenever possible
- Ads on marketplace forums & directory announcement forums (Sitepoint Marketplace Advertising forum, DP Marketplace, etc.)
- Write directory related articles w/ links and submit to article distribution sites
- Trade links with other directory owners
- Provide a free eBook, product, service, etc. with your links, images, directory names as part of the promotionals