If you're talking about the Yahoo Search Engine and SEO, any well placed do follow links on various sites, forums, social media, etc. can help.
yahoo is bing powered, and from my experiements it's much easier to rank on bing and therfore yahoo than it is on google. Just use the same high quality backlink methods you would from google, like quality blogs(.edu and .gov are the best), forums, yahoo answers(need level two to post links), social bookmarking sites(reddit is great, digg, stumble upon, etc...) article marketing sites, etc...you want dofollow and nofollow backlinks in your link profile because it helps make them look natural and legit.
Even i would like to know how to place a link in the yahoo answers. I have tried several times but my link is not showing as link, it has been rendered as a normal text (not hyper text).