Yahoo follows a different algorithm when compared to the ranking of Google.
Here are some useful tips to improve ranking in Yahoo.

1.Including a listing in yahoo directory is useful.Visit this location https://siteexplorer.search.yahoo.com/submit and submit your site.
2.Submit your website sitemap to Yahoo.If your site has got an XML sitemap,visit the location
https://siteexplorer.search.yahoo.com/submit and enter the URL of sitemap.
3.In order to ensure immediate crawling of your website,submit your website RSS to yahoo.Visit the location
http://publisher.yahoo.com/rss_guide/submit.php and add your site URL.
4.Focus more on the keyword density of the site contents.The frequency of the keywords in an important factor.
5.The site contents as well as the title needs to be rich with relevant keywords.
6.Most important is trying to get as many backlinks as possible.

Check the content from one of the best seo forum in http://forums.seochat.com/yahoo-search-optimization-60/how-can-i-improve...