The best answer is to rephrase the question to, what is the best plan to get traffic to a site? What are the best needs for the dite? One can talk about content, but if the site is primarily e-commerce or shop kind of site, where is there room for content? If you only concentrate on search engines for traffic, that could be construed as web spam by Google. A good mix of SEO, referral traffic and even offline promotion may be the best bet.