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2. Have a decent PR. Most site submissions are really from people that are looking to get PR passed to their site.
3. Advertise on Adwords and Overture for terms that are relevant to your link directory. This will be seen by the people that own sites relevant to your directory and will in turn generate link submissions. You can check this by analyzing the conversion rates of clicks to sign up.
4. Optimize your pages so that they appear in top search results for the relevant keywords. This will generate targetted traffic to your link members and the word will spread around, plus possible link submitters will see your good rankings in their search terms and want to submit.
5. Go to webmaster forums like SitePoint where they have boards dedicated to exchanging links. Many times you see people asking for others to submit sites to their directory.