Now that Yahoo has cut the ties with Google for a while, my opinion, based on my own results, is that optimizing for Yahoo is similar to the way we used to optimize for Google (before November, 2003). In other words, Yahoo is more focused on on-page optimization with fairly high keyword density, as well as keywords in title, meta tags (yes meta tags), and H1, H2, etc. headings. It also considers off-page optimization, i.e. backlinks, but not to the same degree as Google does.
Google, on the other hand, is now most concerned with off-page optimization. Get as many relevant, preferably high-PR backlinks as you can to your page - and make sure the anchor text of those backlinks use your targetted keyword phrase. If this matches your on-page optimization, then you are on the right track with Google.