At least one site was showing 50+ W3 errors...as John said this isn't the only measure of success, but I personally believe that meeting standards are vitally important as a professional web developer.
A quick scan of another site showed it had 583 records in Google...that's a LOT considering that many other sites might only have 15-20. As for improving quality, that requires full SEO analysis as John said, something you either educate yourself about or pay to have done.
Google Webmaster tools and Analytics are a good place to start. Make sure the site is "visible" with webmaster tools and showing no errors. Analytics will tell you where people are coming from, where they're going, and how they travel through. Follow the patterns long enough and you'll figure out which pages need to be improved.
I can't say enough about cross-promoting your sites via other media than just Google. Bing just took over for Yahoo's search...have you used their analyzer tool yet? How about social media? To do a good job, you have to be persistent and consistent, posting inbound links (in a non-spam or annoying manner) regularly. My most successful customers actually get more inbound links from other sites and social media than from Google, by a large margin....but they also never stop trying to improve either avenue.
As I tell every potential client, there's no silver bullet. It takes patience, work, and detail to be on top of the rankings.