Yahoo does not index pages as fast as Google does. My shop in online since mid October 2009 and Google has indexed about 2000 out of my 2600 pages. Yahoo has only indexed about 560 pages so far (don't even wanna talk about bing!!! lame lame lame). Anyway, due to more pages being indexed by Google 90% of my traffic comes from Google but I can see a slowly but steadily increasing traffic from Yahoo too. Not yet as much as I wished for, but I think it just takes time for new websites to get a good traffic from all search engines. What concerns Yahoo Answers so I have spent lots of time answering those sometimes really weird questions even a 5th grader actually could answer, but the traffic resulting out of those efforts is almost not worth the time. I rather would suggest to invest the time in SEO work of your website and to do some more keyword research. That for sure will also increase the traffic from Yahoo.
Yahoo cares about the number of backlinks in total, even lame or dropped domain links, and keywords in the page titles. This has been said above by few guys but the point is trying to over optimize for Yahoo mite hurt ur Google position.
Some people use the yahoo directory, but I feel like the more cheaper method is to just advertise on blogs, sites, and forums.