Article directories have been a great source of back-links and visibility. Am writing this article as am running an article directory myself for the last few years, and I do feel that this section of the web needs lots of improvement. The following tips will not only help the article directories but also it will help the article submitters.

1)Technorati Indexed: Get your directories indexed and claimed at Technorati, that way people submitting their articles will get Technorati votes and authority.
2)Do you accept those articles with links inside the article, this might be a risk, as Google might find this as a paid post and give your article directory a penalty. Even if it's not your fault as the article if free so no paid links stuff but you still don't want to get penalty due to poor and in-efficient google's algo.
3)Is your site properly cashed? Check your last 50, 100 or 200 article, and see if they are even indexed by Google or not. When was the last time Google bot gave your site a visit. This problem is mainly because article directories don't add unique content, so search engines don't find anything useful in these sites and stop visiting frequently. So I suggest article directory owners to buy unique and high-quality articles from time to time, so that your sites get properly indexed.
4)Do you accept every article that is submitted? So far I have seen that many submitters use article-spinning software and submit those articles to directories, this is the reason why I say that at least read a few lines from the article before approving it.
5)Research on the keywords for the articles submitted. Then re-write or at least edit the title of the article with a good, low competitive keyword. That way you will have some occasional traffic coming from every article.
6)Get your article directory some quality back links; try social bookmarking if you want free back links. That way your site will gain authority and your over all traffic will increase, as you will be beating your competitors on competitive keywords.
7)Keep adding content yourself, not a bad idea. Just do some research for low competitive and high searched topics and add some small articles by yourself, this way your site will have unique content and your writing ability will develop which will help you in the long run.

Promotion is the key. If you have a good site with good content, but it is not promoted then it won't get traffic. Do social bookmarking, directory submissions; press release submission and distribution, forum signatures; blog posts etc. do it slowly and continuously. If you have a budget for promotions then things will become lot easier, else you will have to do the hard work.