For Yahoo important thing is choose right keywords. For my experience some keywords that give me very good traffic from google , give me almost nothing from Yahoo. So its all depends on niche you are working with . For example I find out that Yahoo have very good amount of celebrity traffic , even more then google sometimes.
Not many people use Yahoo, so don't worry about it, build up good incomming traffic from Google before you start to think about Yahoo and what not, as long as you get traffic from Google then your doing something right.