A listing in Yahoo can boost your site’s page rankings exponentially but unfortunately it is one of the most finicky and difficult sites to get into. For one thing they don’t allow you to piggyback by using well-known keywords or brand names. It is all original, you and manually arranged.
First of all make sure your site is done completely before you submit. Your site will be blacklisted before it is listed if it is not 100% up and running. That "under construction." Page turns Yahoo right off. A site with a clean basic design and lots of good content (is more likely to get inside the Yahoo club than a site full of bells and whistles.
Another caveat: Yahoo requires that the site list the physical address of the business somewhere on the site. This address must be a physical one, they do not accept post office box type addresses. This is one way they keep scam artists out of their search engines. The idea is that only people who are planning to rip off others have hotmail addresses and post office boxes.
Whatever you do, do NOT bombard Yahoo with submissions. If you apply more than once a month, they'll ignore you and your children and your children's children. Another good way to get snubbed by Yahoo is to submit a site to a regional index that has nothing to do with that region, or isn't really a regionally limited site. This is a form of misinformation that is on par with fraud for them.
Whatever you do don't try to sneak multiple listing into Yahoo. By this I mean, don't try to make a free application into a paid category. If they catch you, Yahoo will ban you for life! Follow Yahoo's instructions or else! If you try to sneak around their rules they will get you.