If you are ranking top in Google and MSN then surely you must also be in Yahoo too....
anyway i will suggest you to do following :
*)Use keyword enriched titles on each page of the site.
*)Place keyword enriched text in the first paragraphs of your site-copy.
*)Add a sitemap page and be certain there is a link from the index page to the sitemap.
*)Use HREF links to direct Slurp through each page of the site.
*)Use robot.txt files to keep Slurp out of your shopping cart or log in pages.
*)Acquire strong, relevant incoming links from sites with topics similar to yours.
*)Update your site frequently.