Paying Google for ads. 😛
In a little more seriousness (not that I wasn't a LITTLE serious above), the larger SE's have site map submissions available. Esp. for E-commerce or any large amount of generated content of value, you should have that.
Apart from that, the old "accurate page titles" and "useful meta tags" is good stuff. Tag the canonical version (mobile, desktop or both) of pages. Don't keyword stuff. Make sure you have links from page to page.
After that it's the huge morass of PageRank, backlinks, and all that stuff that is a rather unseemly side of things (although not nearly as unseemly as what's NOT found on the Web).
